I have yet to find a way to copy mp3's to the HD. I have not tried WMA though. Anyone? I could understand mp3's since technically they would require a conversion which may not be in the media player onboard the 360. Your best bet, buy a cheap hard drive they are down to like 50 bucks now for one or less. I have several just lying around. Goto ebay, and get cheap enclosure less than 30 bucks. Then you have 60 gigs or more of mp3s which you can plug into the xbox with usb. Kust make sure to format with fat32.
Thats what I did, in addition to streaming from pc.
Allthough I hope M$ is reading these posts, their streaming and playlist creation, overall video streaming is certainly in need of some improvement especially if you're used to XMBC.