Some of my audio (mp3) downloads will not play in iTunes (I'm running limewire 3.6 beta and the latest version of iTunes)

The strange thing is that they will play fine in the limewire player but when i attempt to either add them to iTunes of launch them in iTunes they just won't open - when i try to launch them with the Quicktime player I get an error message that says:
"couldn't open file xxx because it is not a file quicktime understands.

I've updated to the very latest version of Quicktime for OSX and tried to find an application that might convert whatever format these mp3's are into an iTunes compatible file but no luck.......



HELP please.