I too have been encountering that problem as of late. Any other type of file that I try and download will do just fine, however it seems that all mp3 files are suddenly being corrupted as soon as they start downloading. (I try playing them as soon as they start and get the same error message.)
I've been using Limewire for a number of years without problems now, and my computer is OS 9.2.2. For me it started about two or three days ago, every time I go to click on a newly downloaded MP3 file I get the same error message saying it's not a file that Quicktime understands. Now, any file I downloaded in the past is still there and in working order. It's just anything downloaded since I first got the error message that's unplayable. I've tried playing them in iTunes and a sound program I found called "SoundAppPPC" but to no avail.
I've tried deleting the program and reinstalling it...deleting the program and my Incomplete and Shared Folders and reinstalling it...deleting the program, the two folders and three preferences I found for the program labeled "File Sharing CP Prefs" (or at least, I presume they are affiliated with Limewire) and reinstalling it... but obviously somethings still wrong because I still have the same problem.
Perhaps I am being paranoid here, but I wonder if we've stumbled across some kind of virus for the Mac which quietly and effectively corrupts every MP3 download and leaves all else alone. I don't have anything in the way of virus protection so I haven't been able to scan for viruses, but if you do please try.
Sorry I'm not much more help, but I'm also eager to figure this thing out.