Lime wire doesn't effect teh sound quality of the files, it's simply a transport method like ftp, http, e-mail, dcc, etc. Some of the files shared through gnutella or teh competeting technologies are just bad qualiyt. You just have ot try to find a different file.

I use winamp and it has the best sound quality I've found, I don't know if they make a mac version though www.winamp.com (probably not considering the name).