Some wall warts, as it turns out, are pretty simple inside. This dead one from an old set of speakers, is just one big transformer inside that case. According to the label, it just takes 120VAC and transforms it down to 15VAC.
The primary side of the transformer measures fine with continuity between the two leads. On the secondary side though, my DMM gives me about 50Ω resistance between the two leads. I’m guessing that’s probably why it was discarded as dead.
It will be interesting to see what an AC->DC wall wart looks like inside.