Just wanted to drop a note about presto. This morning I decided to try mp3wrap to contatenate some of my audiobooks into single mp3 files. I was only concatenating the mp3 files, and not text or iamges. The output mp3 file would not play in doggcatcher with variable speed enabled, and one of these files actually crashed the app.
I can play the file if I disable variable speed playback. This is undesirable though, so for now I'm not going to use mp3wrap.