I used to connect my Mac to a PLAY:5 using a cable from the headphone jack to the line in on the PLAY:5. But I was unaware the headphone jack on the iMac was an input. Most folks would be going from the headphone jack to the Amp’s line in.
in that latter case, you would be using the Sonos analog line in, which does always incur a minimum of 75ms, which would be problematic for lip sync. This is why Sonos doesn’t market any of its speakers as computer speakers.
But in your unusual case, the Sonos Amp to the iMac, there are lots of other questions. What is the source that you’re sending from the Amp to the iMac? Where does the video come from? I’m honestly confused.