http://kadath.org/media/sound+LOD%20bug%20demo.wmv (3MB)
I recorded that in 1.5.9, and it is still happening in 1.5.10...
For some reason, these objects vanish and I don't hear them llSay()ing anything if I get more than about 20 meters from them. (They move around.) If I am close enough to see them, and I zoom out, they don't disappear, so it is proximity with the avatar rather than where the camera is.
Also demonstrates volume burst bug, right at the beginning. These objects are near the Balance rezzing area, about 50 meters up. I'm going to leave them for a while. Anyone else want to try and see if they get the same thing?