A new MP3 Player for Windows Mobile is under development over at xda-developers. The program is called Nitrogen and it already received a lot of positive feedback.
Below is a quote from the creator, microfi, that describes the program:
Microfi Nitrogen is a native Windows Mobile application (unmanaged code) that allows you to play MP3 songs stored on your Pocket PC device. It features a file browser, full skin support, “Shuffle” and “Repeat” modes, a 10-band equalizer with preset support, a built-in sleep mode and a command to blank screen to prevent energy waste while playing music. Nitrogen uses the MAD decoder, which is opensource. As the official releases of MAD for Pocket PC seem to be discontinued, Nitrogen uses the libraries from an opensource player (GSPlayer).
Nitrogen was build due to the lack of finger-friendly players for Pocket PC (you have to use stylus even for simple tasks). It features large buttons and you can browse among your playlist by drawing circles on the screen (tapping in the appropriate space). In future releases the program will be expected to have other extensions support and more features, but such things are still being tested to ensure stability and efficiency.