May I mention mhWaveEdit: https://gna.org/projects/mhwaveedit
It's a very straightforward (thus really easy to use) single-file editor
with jack support. It loads and saves mp3s.
I use it all the time to cut/trim/normalise fade single files when I
don't need to do multi-track end funky stuff (with ardour) and I like to
mentionet every now and then on the list as I think it deserves a little