I've had a similar issue almost all of my music vanished except for a few items and oddly when i try to reinstall the MP3's they won't show up in Android's music player but they will in Android Assistant and in Terminal emulator when i do an ls -l or ls -a command in my music folder. Sounds like a bug as i converted everything to MP3 again and reinstalled and yet Android's music player will not see them and neither will Winamp. Going to try to actually mount my SDcard on my Linux box and install the MP3's i made from my FLAC collection as i mentioned in another thread but this still looks like a "boooops" on either Dell or Android's part...i suspect the former but hey it happens. It's very annoying but i have two weeks left on my 30 day return from Best Buy if this isn't fixed then i'll get an HTC.
Edit - Okay 3/15 i formatted my flash card and i am reinstalling my MP3's with Winamp which has built in Nullsoft software that should work with Android just fine. It's a slow process but if it works that could be something of a fix.
3/16 - Okay no go there. Winamp says the files transfered over and shows me with less space on my card but when i put the card in my phone they magically vanish. Either it's something with Windows 7 or Android is not seeing them.