Well Alan, your paritially right about conservation of energy. You miss some details though. You see wood is a good insulator, which means that the sound travels through the tree, gets convereted to heat throughout the tree, but the heat can't get back out. Now the tree is in danger from all the heat inside, it isn't letting it out (except for a bit near the outside which we can ignore) The wood inside therefore has to turn that heat into something else, lest it start on fire. The something that it gets converted into isn't fully understood by me (hey, I'll bet you don't really understand chaos theory either) but when burned the sound comes back. I tried to simplify this the first time, but obviously some of you want more details.