yeah i'm having the same problem, i regret getting the new version. my files open up in real player when i launch them, before the upgrade it was itunes.
i don't know whats going on.

but there is something that can be done until LimeWire fixes this.

first don't launch them from within Limewire. Open Itunes and under Advanced go 'convert to MP3'. this might seem stupid that your converting a mp3 to a mp3 but it works. select the Mp3 file that you downloaded and and what itunes will do is make a copy of the Mp3 and put it in your itunes music folder as Itunes Mp3. then you can trash the other one.