Hm... that's strange. <img src="/ubbthreads/images/graemlins/think.gif" alt="" /> I'm not sure what else to suggest. At least it occurs infrequently.
Part way through the game, one person had the sound turn into a high pitched buzz when underground, causing them to turn the speakers off for dungeons, caves and cellars. Later in the game the sound suddenly returned to normal, for no apparent reason.