Yeh don't use Audacity lol - for this to be done seamlessly you need be able to zoom in like a bagillion times on the places where you want to cut and stitch togther to make sure the waveforms match up exactly. Not sure if Audacity can do this but i know on Goldwave you can
gonna be very difficult to replace a tune in a mix in audacity, but not impossible.
you would have to open the mix in audacity, change the tempo of the mp3 you intend to insert
over the top so that it syncs perfectly (by ear and trial and error) cue it before your bad mix on a new
audio channel and automate the levels to mix in the new track and back to your original
(i'm not even sure this is possible without trying it myself).
it would be easier in a more advanced editor like Soundbooth, but where there is a will there is a way.
i would suggest that it would be less tricky and time consuming re-recording the mix.
practice makes perfect!
would be interested to know if you do manage to do this, perhaps you could post a walk through.
Start the track you had playing before you started mixing in the track you fucked up with.
After you finish, go back and find a part of the first track that doesn't have too much going on, zoom in and find a "landmark", then go to that same part in the good mix. Highlight everything in between and DELETE. Now all the bad stuff is gone. Give it a listen and ensure there is no noticable skip/jump/etc.