So a bunch of CDLC I have use pretty shitty audio, I was wondering if I can switch out audio files for the songs.

Changing the audio file is the easy bit. Just break it up with the toolkit, replace .mp3 and rebuild. The Hard bit it getting the new audio file to line up with the song. If you manage to find a better sounding version of the same performance, or of a performance played at exactly the same tempo, and you can edit it to start at exactly the same place as the original, then no problem.

The toolkit I'm referring to can be got from the link at the top of this page. Under the Tools menu you want Rocksmith Custom Song Toolkit.

When you run it and open an existing .psarc it asks where you want the project created and splits the .psarc into all the files that it is composed of. You can root around in there for the mp3 file. Beware there is a tiny mp3 used for the preview. You need to replace the bigger one.

It's a while since I've done this so I can't give specific steps from memory.

tl;dr. It's possible but quite tricky.