Issue in returning an audio file from ASP.NET web service

I'm trying to return an audio file from a web service application using ASP.NET. Following is the code that i'm using:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Media;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Web;
using System.Web.Services;

namespace WebApplication1
{
    /// <summary>
    /// Summary description for WebService1
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    // [System.Web.Script.Services.ScriptService]
    public class WebService1 : System.Web.Services.WebService
    {

        public HttpResponseMessage Post()
        {
            var path = @"C:\test\a.wav";
            HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
            var stream = new FileStream(path, FileMode.Open);
            result.Content = new StreamContent(stream);
            result.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/octet-stream");
            return result;
        }

    }
}

I found the above code from another question at Returning binary file from controller in ASP.NET Web API

I'm using Visual studio 2012 with target framework .Net 4 for this application. When I build and view this web service, I get the following error:

Server Error in '/WebApplication1' Application.

To be XML serializable, types which inherit from IEnumerable must have an implementation of Add(System.Object) at all levels of their inheritance hierarchy. System.Net.Http.Headers.HttpContentHeaders does not implement Add(System.Object).

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: To be XML serializable, types which inherit from IEnumerable must have an implementation of Add(System.Object) at all levels of their inheritance hierarchy. System.Net.Http.Headers.HttpContentHeaders does not implement Add(System.Object).

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.34274

Kindly, let me know where might be the issue and how can I resolve it.