Ah too bad.. in Panther you could select a buncha file and Get Info on them and end up with all of them bunched together for this. Unfortunately, this was changed in Tiger back to the way it was in Jaguar, and now each file opens individually, so the easy way is blocked off.
Hopefully this will work:
I hope all your MP3s are in one single directory, I'm not sure how to make this recursive for multiple subdirectories.
Instead, you'll have to use an XCode tool to fix it.. If you haven't installed XCode, the you will have to. Then you will have to open the Terminal.app and do/type this:
sudo /Developers/Tools/SetFile -a L /Path/To/Your/MP3s/*.mp3
Where naturally, "/Path/To/Your/MP3s/" is replaced by the actual path to your mp3s. Something along the lines of: ~/Music/Moved_MP3s/*.mp3
The easiest way, if you're unfamiliar with the Terminal, will be to do this part "sudo /Developers/Tools/SetFile -a L " (note: there's an important space after the capital L), and then drag the folder containing your locked mp3s into the Terminal window, and it will automatically fill in the correct path, you just need to make sure you add "*.mp3" to the end so it will only set the L flag (or unset if it's already set) on mp3s.
There may be an easier/better way, but I'm not familar with it, so you might want to wait and see if someone chimes in with a better idea.