I know the question is in C++, but here is good point on this.. Also like you say, the code that works for your desk also can work on the Pocket PC.
So I worked Windows Mobile app don in C#, that had a reminder feature and we used the wmplib (Windows Media Player) Library to play songs (mp3 included).
First you need to add the wmp.dll to the references, found in c:\Windows\System32 (or what ever is your windows directory). Then you just need to code it like this:
private WMPLib.WindowsMediaPlayer player = new WMPLib.WindowsMediaPlayer();
// url is the path of the file
private void PlayFile(String url)
{
player = new WMPLib.WindowsMediaPlayer();
player.URL = url;
player.settings.volume = 100;
player.controls.play();
}
Here is the reference for this code
And for the C++ here you can find how to do it