I use Adobe Audition software (which is based on the old Cool Edit program that Adobe bought some years back). It has a "clip restoration" feature that I've used on some heavily clipped material. It certainly doesn't "fix" a song, but it can make it "better."
In looking at the SeeDeClip web site, this particular function appears almost identical.
The X11 is an open source graphics program (most modern Linux versions come with it installed) but it shouldn't be a problem for Apple Macs. (Actually, I read the installation instructions and they seem to make the X11 requirement pretty clear.)