I bought a playbase and bring back to Taiwan for a while. Few weeks ago, my playbase was no sound from optical port. I read many story here and it seems many people has same issue here and no good solution. I also reset my playbase many time and useless.
Finally, I decide to open it on my weekend and follow the setps by steps from https://zh.ifixit.com/Guide/Sonos+Playbase+Optical+Audio+Port+Replacement/114606.
when I open it and check the optical cable, I find the socket to PC board seems loose. It is almost disconnected. so I replug it again and test it. The sound comes back perfectly. now I enjoy the movie and music, hopefully this experience help. one more remind, you need patient and very careful to open the case.