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.
DM