Well, originally, the default music player detected all of the music on my card. I recently installed a 16GB card in it, with the contents of my 8GB card copied over. After that, it stopped detecting the same music. I changed the name of the card to be the exact same, and everything. I'm almost down to formatting the card and trying it again. If all else fails, I'm considering putting my music on the n900 itself, since there is plenty of room.

Did you try restarting the n900? o.o

It's been restarted multiple times since then.

Did you also try starting the mp3 files from a file browser or xterm?

Either way it does sound your card needs reformatting. It's just wiser that way...

does it work if you open the songs from file manager? and have you tried other apps like instinctiv and mediabox?

The files open perfectly fine from file manager, but neither the default media player nor mediabox will automatically detect them. I'm going ot reinstall my old card and see if they start picking them up. If they do, I'm reformatting the card if it isn't in the same format.

You can also try reinstalling the extra codecs support.

Maybe this could help http://talk.maemo.org/showthread.php?t=49752
according to this thread theres some errors with 16GB card...