Code Behind:

using System;
using System.Configuration;
using System.Data;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Net;
using System.Xml;

public partial class _Default : System.Web.UI.Page 
{
    // Bing API Id: get your own @ http://www.bing.com/developers/appids.aspx
    const string AppId = "FD087E7F3CDFEC530B63A2AC2FEEC5846127DA39";

    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        // retrieve the basic parameters
        string sourceText = TextBox1.Text;
        string targetLang = DropDownList1.SelectedValue;

        // Sample code from the Bing API documentation, with minor modifications
        // see http://msdn.microsoft.com/en-us/library/dd877832.aspx
        HttpWebRequest request = BuildRequest(sourceText, targetLang);

        try
        {
            // Send the request; display the response.
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            TextBox2.Text = DisplayResponse(response);
        }
        catch (WebException ex)
        {
            // An exception occurred while accessing the network.
            TextBox2.Text = ex.Message;
        }
    }

    static HttpWebRequest BuildRequest(string sourceText, string targetLang)
    {
        string requestString = "http://api.bing.net/xml.aspx?"

            // Common request fields (required)
            + "AppId=" + AppId
            + "&Query=" + HttpUtility.UrlEncode(sourceText)
            + "&Sources=Translation"

            // Common request fields (optional)
            + "&Version=2.2"

            // SourceType-specific request fields (required)
            + "&Translation.SourceLanguage=en"
            + "&Translation.TargetLanguage=" + targetLang;

        // Create and initialize the request.
        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(
            requestString);

        return request;
    }

    static string DisplayResponse(HttpWebResponse response)
    {
        // Load the response into an XmlDocument.
        XmlDocument document = new XmlDocument();
        document.Load(response.GetResponseStream());

        // Add the default namespace to the namespace manager.
        XmlNamespaceManager nsmgr = new XmlNamespaceManager(
            document.NameTable);
        nsmgr.AddNamespace(
            "api",
            "http://schemas.microsoft.com/LiveSearch/2008/04/XML/element");

        XmlNodeList errors = document.DocumentElement.SelectNodes(
            "./api:Errors/api:Error",
            nsmgr);

        if (errors.Count > 0)
        {
            // There are errors in the response. Display error details.
            return DisplayErrors(errors);
        }
        else
        {
            // There were no errors in the response. Display the
            // Translation results.
            return DisplayResults(document.DocumentElement, nsmgr);
        }
    }

    static string DisplayResults(XmlNode root, XmlNamespaceManager nsmgr)
    {
        string results = String.Empty;

        string version = root.SelectSingleNode("./@Version", nsmgr).InnerText;
        string searchTerms = root.SelectSingleNode(
            "./api:Query/api:SearchTerms",
            nsmgr).InnerText;

        // Add the Translation SourceType namespace to the namespace manager.
        nsmgr.AddNamespace(
            "tra",
            "http://schemas.microsoft.com/LiveSearch/2008/04/XML/translation");

        XmlNodeList nodes = root.SelectNodes(
            "./tra:Translation/tra:Results/tra:TranslationResult",
            nsmgr);

        // Display the Translation results.
        foreach (XmlNode result in nodes)
        {
            results = String.Concat(results, result.SelectSingleNode("./tra:TranslatedTerm",
            nsmgr).InnerText);
        }

        return results;
    }
    
    static string DisplayErrors(XmlNodeList errors)
    {
        string results = String.Empty;

        // Iterate over the list of errors and display error details.
        foreach (XmlNode error in errors)
        {
            foreach (XmlNode detail in error.ChildNodes)
            {
                results = String.Concat(detail.Name + ": " + detail.InnerText);
            }
        }

        return results;
    }
}

Client Code:

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="TranslateUsingBingSpeechSample.aspx.cs" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Translate to Speech Sample</title>
    <style type="text/css">
        .style1
        {
            font-family: Calibri;
            font-size: small;
        }
        #Text1
        {
            width: 463px;
        }
        .style2
        {
            width: 1111px;
            font-family: Calibri;
            font-size: medium;
        }
        .style3
        {
            font-family: Calibri;
            font-size: small;
            width: 1111px;
        }
        .style5
        {
            width: 71%;
        }
        .style7
        {
            font-family: Calibri;
            font-size: medium;
            width: 500px;
        }
        .style9
        {
            font-family: Calibri;
            font-size: medium;
            width: 573px;
        }
        </style>
</head>
<body>
    <form id="form1" runat="server">
    <table style="width: 800px">
    <tr>
        <td class="style2" valign="bottom">
            <table class="style5">
                <tr>
                    <td>
                                <img onclick="document.location='http://www.h2ktech.net';" style="cursor:pointer;width: 76px; height: 66px" alt="" title="http://www.h2ktech.net" align="left" src="http://www.h2ktech.net/images/H2Kcredits.png" /></td>
                    <td class="style7">
                        Translate2Speech Sample Using the Bing API - ASPX</td>
                </tr>
            </table>
        </td></tr>
    <tr><td class="style2">&nbsp;</td></tr>
    <tr><td class="style2" valign="middle" >Translate&nbsp;&nbsp;&nbsp;&nbsp;
        <asp:TextBox ID="TextBox1" runat="server" Width="519px"></asp:TextBox>
&nbsp;to&nbsp;
        <asp:DropDownList ID="DropDownList1" runat="server">
            <asp:ListItem Value="fr">French</asp:ListItem>
            <asp:ListItem Value="de">German</asp:ListItem>
            <asp:ListItem Value="es">Spanish</asp:ListItem>
        </asp:DropDownList>
&nbsp;&nbsp;&nbsp;&nbsp;
        <asp:Button ID="Button1" runat="server" Text="Go !" onclick="Button1_Click" />
            </td></tr>
    <tr><td class="style3" style="font-size: medium" align="left" valign="middle">
        &nbsp;</td>
    </tr>
    <tr><td class="style3" style="font-size: medium" align="left" valign="middle">
                Translation&nbsp;
        <asp:TextBox ID="TextBox2" runat="server" Width="519px" ReadOnly="True" 
            ></asp:TextBox>
        &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
                </td>
    </tr>
    <tr><td class="style3" style="font-size: medium" align="left" valign="middle">
        &nbsp;</td>
    </tr>
    <tr><td class="style2" align="left">
                <table>
                <tr>
                    <td class="style9">
    <iframe id="playBody" title="play wave" frameborder="0" scrolling="no" 
        style="width: 300pt; height: 38px; border-style: none; margin-bottom: 0px;" 
            name="playBody"></iframe></td>
                    <td>
<span class="style1" title="Automatic translation can help you understand the gist of the translated text but is no substitute for a professional human translator."><u 
            style="font-size: small">hover over for mt 
        disclaimer</u></span>
        </td>
            </tr>
            </table>
    
        </td></tr>
    </table>
    
    <script language="javascript" type="text/javascript" >
    
            // if we have any translation, invoke the Speech feature
            var translation = document.getElementById('TextBox2');
    
            if (translation.value != '')
                {
                var url='';
                var language = document.getElementById('DropDownList1');
                var source = document.getElementById('TextBox1');
            
                if (source.value != '')
                {
                    url='http://www.h2ktech.net/translatespeak/text2speech.aspx?from=en&to=' + language.value + '&text=' + source.value.replace(/ /g,'+');

                    var iframe = document.getElementById('playBody');
                    var doc = iframe.document;
                    
                    if(iframe.contentDocument)
                    {
                        doc = iframe.contentDocument; // For NS6
                    }
                    else if(iframe.contentWindow)
                    {
                        doc = iframe.contentWindow.document; // For IE5.5 and IE6
                     }
                     
                    // Put the content in the iframe
                    doc.open();
                    doc.writeln("<font style='color: #000000; font-family: Calibri;font-size: small'>click <a href='" + url + "'>here</a> if you get no audio </font>");
                    doc.writeln("<br/>");
                    doc.writeln("<object id='player' width='1' height='1' classid='CLSID:22d6f312-b0f6-11d0-94ab-0080c74c7e95'");
                    doc.writeln(" codebase='http://activex.microsoft.com/activex/controls/mplayer/en/nsmp2inf.cab#Version=5,1,52,701' type='application/x-oleobject'>");
                    doc.writeln("<param name='fileName' value='" + url + "'>");
                    doc.writeln("<param name='autoStart' value='true'>");
                    doc.writeln("<param name='showControls' value='false'>");
                    doc.writeln("<param name='loop' value='false'>");
                    doc.writeln("<embed type='application/x-mplayer2' src='" + url + "' pluginspage='http://microsoft.com/windows/mediaplayer/en/download/'");
                    doc.writeln(" id='mediaPlayer' name='mediaPlayer' displaysize='4' autosize='-1' showcontrols='false' showtracker='-1' showdisplay='0'");
                    doc.writeln(" showstatusbar='-1' videoborder3d='-1'autostart='true' repeat='1' loop='false'>");
                    doc.writeln("</embed></object>");
                    doc.close(); 
                }
            }
            
    </script>

    </form>

</body>
</html>