I am experienceing a similar problem.
Have two MP3 audio files, both 128kbps, 44kHz mono with very similar, and good, sound quality when played separately. I am trying to merge the two, but when I copy/paste from one file into the other, or to a third empty file, the new segment/s are being processed in some way that is reducing the quality noticeably.
Playing the resulting merged file has the sound quality changing back and forth.
It can't be stereo to mono in my case as the files are all mono to start with, and it can't be default sample rate and channels a new file as I have the same problem when pasting into an existing file with good sound quality.
Is this a known problem, a "feature" of the free copy and rectified in the upgrade version, or is it something I am doing wrong?
John S.