I'm having a bizarre problem on my 2010 Mac Pro after upgrading from 10.13.5 to 10.13.6. I usually use a Logitech USB headset for sound output. After upgrading to 10.13.6, audio quality was significantly worse through the headset. I would describe it is as muffled telephone quality, as if playing back a low-fidelity sound of 11KHz versus high-fidelity at 44KHz+. The problem occurred when playing audio in iTunes, web browsers, system sound effects, and other sources, so it seems all system audio is affected. Now the bizarre part: the problem only occurs when the headset is plugged into the lower USB port on the front of my Mac Pro. Plugging it into the upper USB port, keyboard USB port, and a couple of others that I tried returns audio quality to normal. Other output devices don't have this problem. I tried playing audio through analog headphones (apple earbuds) plugged into the headphone port, through the internal speaker, as well as through my HDMI display's built-in speaker and all have proper audio quality. My workaround is just to use another port, which is fine by me, but any ideas why this is happening or for a permanent fix? I'm wondering if performance of that port will be affected for other devices, as well. I'm thinking of re-installing the update and perhaps filing a bug report.