I think I figured out the "CoSo randomly playing audio, even in Ostrich mode" - it's Ostrich mode. Apparently on Safari (macOS 13.3.1) with Ostrich mode *ON*, it autoplays CoSoCom, twice. And because CoSoCom isn't showing, I can't click the stop button.

With Ostrich mode *OFF*, it autoplays once, and clicking the "Stop" button stops it.