2nR-Musiker Wireless MP3 Client and Server 0.8 Beta
the author of this app says:
2nR-Musiker is a simple client/server application that makes your PC-stored MP3s available to your PocketPC anytime and anywhere. Stream, send and receive MP3s to play on other computers and PocketPCs with the 2nR-Musiker player. You can hook your PocketPC up to your stereo equipment to play your MP3s right from your PC's hard drive!