So is the Onkyo line-out audio (Zone 2) —linked to —> Port RCA line-In …and the Port line-in icon (in the Browse/Music Services Tab in the Sonos App) selected as the playing audio source for the Move? If so, then check the volume level of both the Onkyo Receiver and the Move.

Also note that the line in on those Move 2 speakers is subject to the 75ms delay, which will make them a poor choice as surrounds…one reason, perhaps, that Sonos doesn’t advertise any of their ‘portable’ speakers as for surround use.