When I play a song the playback is fine, when I lock the keyboard it is as well. But when I unlock the keyboard the song volume goes down a lot and I can barely hear it, pausing and resuming fixes it, but is a real hassle. I had this same problem with my old 8530, but with that phone it happened with the speaker phone too. I'm running .337.
Never have heard of this issue nor can I reproduce it. Could just be one of those bugs people tend to have that others don't. Have you tried a battery pull? If that doesn't fix it, I would wipe and reload the OS.
Posted from my CrackBerry at wapforums.crackberry.com