how do i turn off participants join sound (doorbell) when i'm not host or co-host?

Assuming you have write access to the zoom installation directory on your computer (on my linux install, it's under /opt/zoom/), you can delete the files dingdong.pcm and dingdong1.pcm. This finally killed the doorbell sound for me.

For good measure, I also deleted mute.pcm and unmute.pcm, which apparently are responsible for the new mute/unmute sound effects that zoom has apparently decided are mandatory as well.

And just to affirm what others have said, these sounds *used* to be optional. But Zoom apparently decided that everyone needs to be blasted with them involuntarily. It has made many meetings impossible to meaningfully participate in.