How to play an mp3 file into a voice call

Hello everyone,

I've been cruising around the net for some help, then I found this forum. Hope some developer guys can help me on this.
I'm working on my own softphone application, but kind of bumping into some trouble when I try to play a sound file (mp3)
into the call. For seeing it more exactly, you can check the website I used as a guideline: http://www.voip-sip-sdk.com/p_345-voip-mp3-playing-voip.html

Is this the best way for sound playing? Or do you have other suggestions?

Thanx