GSPlayer is one of the few must-have open-source code apps for Pocket PCs for which it's hard - if not impossible - to find a better commercial alternative. As a light-weight audio player it supports playing and streaming various audio formats including MP3, Ogg Vorbis, and Wave. The latest version V2.24 adds support for Windows Mobile 5.0 devices.
You can download GSPlayer (and its source code)
from here.