PalmPlayer

the author of this app says:

PalmPlayer allows MP3 playback and a great interface for your Palm-size PC. Using the Xaudio MP3 engine, PalmPlayer plays back MP3 files with great quality (up to CD quality stereo on the Casio E-100). The interface is fully customizable: skin authors can define how their interface should look artistically as well as functionally--bitmaps and locations of the components can be defined. Skin documentation will be available soon.