Twilio send Mp3 url

I have a problem in c # I want to send the url of the file that should be reproduced in the calls that will be made to people currently works like this.

Twilight Bins

<?xml version="1.0" encoding="UTF-8"?>
<Response> 
    <Say>{{body}}</Say>
    <Play>{{play}}</Play>
</Response>

I sent the information in c #

public async Task<IResponse> CallAsync(string from, string to, string msg)
    {
        var pnFrom = new PhoneNumber(from);
        var pnTo = new PhoneNumber(to);

        var body = WebUtility.UrlEncode(msg);
        //var url = "https://api.twilio.com/Cowbell.mp3";

        var url = "http://compunexs.com:2222/CMD_FILE_MANAGER/domains/compunexs.com/public_html/holamundo/Gallo.mp3";

        var call = await CallResource.CreateAsync(
            pnTo,
            pnFrom,

            url: new Uri($"https://handler.twilio.com/twiml/EH4280c1a3a5e140af2f00b9d076fd8ba9?play={url}"));

        return new CallResponse(call);
    }

You can see that I try to execute an mp3 by sending the path of my file. works correctly sending: var url = "https://api.twilio.com/Cowbell.mp3"; but I do not understand why mine does not.