Is the smell a relatively recent change (and you know it didn't exist last year at same time)? Then my money would be on the dead animal poss. If it comes back every year then it's more likely to be bats because they tend to reappear annually.

Bats, aside from the smell which isn't universal, pose two problems: one is safely getting rid of the guano (aka bat poop) which has some health risks (mainly for breathing) and secondly, having bats in the house offers the small possibility that one will get into the occupied space while you are asleep. If this happens (particularly where children are sleeping) there is the possibility of inadvertent bat/human contact raising the risk of rabies transmission. Teach the kids to stay away from bats and report immediately if they see one. No need to panic if you're absolutely sure the bat has had no contact with humans (or pets). Let the bat out by opening a window and leaving the space. If there's any doubt, then you may have to trap the bat and take it to be euthanized and tested. Bat rabies is nothing to fool around with as in many areas it is the most likely vector for fatal human rabies cases. These are very rare, but do happen. Most bats, by far, are not rabid, and are great beasties to have around, but there is the very small, but real, risk with any bat/human contact. Your county health department would be your resource here to assess local risk and get treatment post exposure.

Since I don't like having anything euthanized, I put a lot of effort into keeping the bats out of my living space. Sadly, I did have to have one tested because it was found in a child's room. It was euthanized, but tested disease free, so I felt doubly bad, particularly because in my area bats of most kinds are suffering a major disease outbreak (of a kind humans can't get, only bats) and their populations are very threatened.

A dead animal in the walls, while not pleasant to think about, is something that will dry up and stop smelling in a few weeks.

One other possibility: an old bee nest in the walls. Sometimes these can smell pretty rank if the honey ferments. This is also self-curing.

Good Luck!