In ZQuest, via the Etc. -> play music menu, certain MP3 files will not play. I suspect it has something to do with the file name, as ZQuest won't play any whose filenames have a space and a hyphen next to each other. For example
MP 3.mp3 will play
M-P3.mp3 will play
M - P3.mp3 will NOT play
Not a major bug, just a glitch if you will, so this shouldn't be too high on the priority list. I have yet to test whether ZC has the same glitch.