Here’s the thing…
I used Mixxx on Windows 10, and yes, I used a USB Mic. Clipping, popping sounds all over the place started happening as recently as last week. I increased the Audio Buffer in Preferences and the voice was delayed but still clipping and popping.
So, I remembered the official stance on USB with Mixxx: don’t use it. (Haven’t had an issue for 4 years. Figured if it works and isn’t supported, why not, right?) In the past, I was able to resolve by unplug/replug/Query Devices/reselect. Now, that doesn’t make a difference. Even a reboot, still pops.
Today, I removed all USB connections (except Keyboard and Mouse) then plugged in an analog headset (stereo/mic jacks, sound card inputs only, no power). The SAME popping sounds are happening. This lends me to think USB isn’t the cause anymore.
I’ll hop into Ubuntu 20.04 on the same machine and let you know if it’s consistent there.