What is your source of music? Are you playing a music service? Does the Volume vary from track to track or within a track?

Spotify, Volume vary all the time ...

Do you have variation with another source, such as Internet Radio stations?

same situation on radio stations 

in the fixed mode works fine but on the pass-through volume goes up and down all the time.

Which model ONKYO? You have two PORTS’s connected as inputs to the ONKYO. Do both exhibit this behavior?

• Onkyo 6100
• 2 Sonos Ports in 2 different inputs

Does the ONKYO indicate any Volume change commands are being executed? Are you using any sort of voice control?

No, Onkyo does not indicate volume changes - no voice control use.

What is the speaker impedance? How loud are you listening? Maybe the ONKYO is entering a protection mode.

I could understand some kind of amplifier protection, but the fluctuation in playback volume doesn’t explain it— speaker impedance at 8 Ohms.

I’m not sure what you are experiencing. The typical complaint is that level can vary from track to track and this is caused by differences in the track recording levels. I don’t recall any previous complaints with respect to levels changing while a track is playing, however, some tracks, particularly classical music and some jazz tracks are very dynamic. Pop music levels typically don’t vary much.

An Internet news radio station would be a good test choice because the audio level will be very consistent, although some stations will boost the level on commercials somewhat.

You could take this up with SONOS phone support. First, submit a diagnostic after one of the Volume level incidents. If a PORT is having issues, they will usually leave data in the diagnostic.