I've recently been having some odd playback problems with my clip+. There are a few songs that cause what seems to be loss of volume in most frequencies, basically everything above bass range. It's generally sort of rather short, random, and constant loss during playback of one or two songs. It only happens during parts of the song, usually when the song is at it's louder parts. At first I thought it might have been the clip itself, but it plays back fine in the default firmware. Volume doesn't seem to make a difference either. The songs also play back fine on my X5L with rockbox, though it's still on 3.6.
The clip is running the current version of rockbox as of this post, it's been having problems with every version I've tried though, all 3.7+.