I have recently been having issues with very low audio levels from my Sonos Connect which plays through speakers via a tuner (at the highest volumes on Sonos and the tuner I could just barely hear music from my speakers).  After some trouble-shooting (including using the loop-through method suggested in support), I isolated the problem as a failure in the RCA ouput.

I spent a fair amount of time with customer support technicians who ran diagnostics several times.  However, those diagnostics apparently did not reveal any problems (exactly what they do is unclear).  Eventually, they wanted me to do various cable re-arrangements etc.  As I had already connected, disconnected, swapped etc. multiple cables multiple times from various devices that was not going to yield any answers.  My conclusion is that there must be some circuity in the Connect that has failed and resulted in the RCA outputs not delivering meaningful signal.  The customer service people were not able to confirm that, but they didn’t seem to even consider it a possibility (I guess it is not on their script).

In any case, the solution turns out to be buying a digital - analog audio converter (including appropriate cords and power adapters)  and using the toslink optical output that is also on the back of the Connect and running that output into the converter and then connecting the RCA cables to the converter output.  For $20 and ten minutes of effort this solves the problem.