In some parts of the world like the EU or India, you should be able to get a full refund with little legal expense. But that's about it.

Surely, you mean within the statutory guarantee period only (2 years in the EU).? 

In India, even after that period expired because Sonos actions stopped my music. EU as well, I suspect. In India the consumer courts are very pro consumer.

PS: One would have to be willing to surrender all the kit, if the manufacturer desires that.

Proving the case would be laborious however, I would imagine. There are so many interacting variables, including end-user error.  I'm not supporting Sonos, but they can still probably afford fancy lawyers! 

Not really; these are less formal places and manufacturers usually pay up because doing that is less than the cost of their fancy lawyers! A consumer can speak for himself, does not need a lawyer. Just a well documented case to put before that court.

Well, good luck to anyone who tries.  Sonos may collapse before any cases reach a court! . I believe the current CEO has previous in this regard

Sonos collapse will only happen if/when they stop finding enough new buyers to support growth in their expenses including cost of stock options..