I've used probably every single music playing software on the market, and I just keep on coming back to this.
http://www.pocketmind.com/pocketmusic.htm
Its pretty much WinAmp on the ppc.
Pocket Player would be my second choice, but it isn't free nor is it very fast.
BTW: I'm using the old version of Pocket Music, not the 5.0 version on the website.