Circling back to say that the issue is resolved. As both @Airgetlam and I suspected, the problem was not in Sonos port but in the (pretty non-intuitive) configuration of the Onkyo AV receiver.
Basically, sending sound from the Onkyo AV receiver (or the attached cassette deck or CD player) is accomplished by, in the event that someone else is looking for this information:
- Onkyo’s Zone 2 output → Port’s line in (RCA)
- Making Onkyo aware of Sonos port by having them both on the same wifi network and using Onkyo’s Hardware → Works with SONOS setup options.
- When one wants to actually send sound to the speakers, turn the Onkyo AV recevier on, use the Zone 2 button on the front of the receiver, then pick the source to send through Zone 2 by using one of the selectors on the front of the Onkyo (tuner, CD, NET, etc.) and then using the ‘Enter’ button on the front of the Onkyo receiver.
- One decides whether to control volume from the Onkyo receiver (Onkyo Line out ‘Fixed’ rather than variable) or using Sonos (either on the speakers or using the Sonos controlloer app).
Kindy of kludgy but it works. I can now stream through SONOS port to my SONOS speakers and use the iPhone app to determine which speakers or speaker groups to use.
And my thanks to the woman who saw my post, contacted me, and walked me through it. There’s a place in heaven for people who do that.