I encountered the same problem, and as I understood it, SONOS is indifferent to the requests of its customers.
I connect TRAKTOR S2 MK3 through Line-in and so it is absolutely impossible to play. For me, it’s really not clear why this problem cannot be solved for so many years, because it is fixed by one new firmware. Why do we need smart chips if you plugged the speaker through the cable? Turn them off and let the sound go directly!
Dear and beloved company SONOS, forget about it or is there at least some chance in the future? After all, there would not be this problem, many would choose PLAY 5 as a monitor, buy a second PLAY 5 speaker for themselves.