I ran the load test, raising the port count slowly (every 5 to 10 minutes or so) to 100, 150, 200, 250, 275 and 300 ports.
There were no audio problems until I reached 275 ports. After probably 10 minutes at 275 ports, audio problems started, and after that they were consistent.
Of course I can’t guarantee that there were no audio problems at lower port counts - if the problem only occurred intermittently I might not have caught it or I might not have waited long enough.
Once the audio problems started, I slowly lowered the number of ports back to 250 ports. The audio problems disappeared after about 2 1/2 minutes at 250 ports.
I then increased the number of ports back to 275 ports.