Last week, I mentioned the Modula engine for sound and MOD file playback. The Palm OS gaming uberdeveloper Astraware has its sound engine, Aurora, available for licensing to other developers. It's a small library that supports both the standard Palm OS 5.1 streaming sound APIs and the Yamaha PA1 sound chip used on many Sony CLIE devices. In addition to supporting wave file playback, Aurora also handle music in ScreamTracker format.
By the way, Astraware also has a nice whitepaper on streaming audio support posted to their site, talking about historical and current sound formats and the overhead of supporting different options.