Just tried to mute sound on macOS (in browser, Firefox 81) by pressing Cmd+M. Turns out that:

1) You need to use Ctrl+M even on macOS.
2) Cmd+M does not do anything except pressing and not releasing the X key. That makes games that use the X key unsolvable.

The first one, we can remember. But the second one is a bug to fix.

hmmm... as someone who uses the same software (I think?) I tried it... I was doing the cmd+M and It let me use the "X" key afterwards..? IDK I'm sure that there might be jank with the in-frame player though

Yeah, the standalone PICO-8 app just ignores Cmd+M. I'm talking about the in-browser version.