I'm not saying what they did was ok, but would it be possible to solder on a raspberry pi or something? Compared to other types of board rework, audio connections and DC power is relatively easy.
I'd suspect that there is. That kind of ruins the point of Sonos, though - their appeal was their ability to do a lot of different things, automatically, with just a wifi connection. Having to set up some sort of alternate source of audio means that I really should have just bought a cheaper speaker back when I bought my Sonos products.