Not fixed yet.
Turning on QoS at the switch port has not fixed the the audio problem on the Cisco phone when the iMac turns on. The technician has made some other changes but we still face the same problem. I had them make the same changes to the switch port that goes to my Cisco phone and I will see it solves the problem for me. So I am not sure if we have a problem limited to a particular station or if this is an office-wide problem.
Fortunately staff don't power up their Macs multiple times a day.