Xfinity audio remains when TV is switched to other streaming sources

All of these played well together until very recently: while on Xfinity I could switch to Netflix or Prime or Apple by voice command to the Xfinity remote OR by using the Roku remote to bring up the TV's home screen and select N/P/A. I used only the Roku TV switch for PleX, as Xfinity does not allow choice of a server. In all cases, I got the appropriate audio as well as video.

Starting about ten days ago, with no hardware or setting changes, if I switch from Xfinity to N/P/A by voice command to the Xfinity remote, all is well. But if I switch using the Roku TV interface, I get the video I want but BUT STILL GET THE XFINITY AUDIO CHANNEL non-stop along with video from the other four streaming sources. I have unplugged shut down and restarted the Xfinity gateway, the DVR, and the TV; factory reset the Denon; tried other HDMI cables, all with no luck.

The problem disappears if I run HDMI straight from the DVR to HDMI 4 (eARC), but then I lose the 5.1 sound system and have only the TV's speakers. More to the point, the DVR -> AVR -> smart TV path worked perfectly for a long time and stopped working for no reason I can see. 

Any ideas?