In short, no. I'm going to assume that you mean mp3 files. Once an mp3 has been compressed, parts of the sound (that the human ear supposedly can't hear) have been removed, there really is no way to get them back. The same goes for all lossy audio formats (Mpc, ogg, mp3 pro, wma etc).
If you have the audio in a lossless format like Wave, Monkeys Audio (.ape) or Flac, then it's generally already at it's full quality, so no need to worry there
I assume that you realize upping the bitrate of an mp3/ogg etc wont actually improve the quality, so i'll move on
. The only option you really have for improving an mp3 file is getting an audio editor like Cool Edit, or Sound Forge (Which is what I use). You can try eqing it, or applying some effects, but realistically, you're stuck with what you've got. Sorry I couldn't be of more use