I am bumping this because I have had this issue for about a week now and it is driving me crazy! I only get the unwanted sound when I am in my SL home/land. It is SUPER annoying. My sound is the Mario Super Brothers video game sound bite. It plays at random times with not a lot of time in between and it plays no matter what is muted. UGH! I am going to try the one suggestion from other about clearing my cache and hope that works for me as well.