answer:When I looked into this a couple of years (I ultimately decided NOT to use the low voltage stuff) I was told that it was not advisable to put the transformer into the wall. The reasoning (what I was told) is that (unlike the actual wire) a transformer can, over time, deteriorate, stop working, potentially catch fire, and in the worst case explode. Not that it happens frequently, but it can and does happen. If the worst were to happen, you would have a fire going on behind a wall and it would be damnably hard to detect and put out. Even if the best happens – the transformer dies quietly – you would still have to cut the wall to replace it. We didn’t talk ceilings, because I was turned off from the risk at that point.