I've just used the music() command (other than the game loop, that was the extent of my code) to play two music tracks (with 7 sfx).

function _init()
 music()
end

function _update()
end

function _draw()
end

When I came back to the sfx editor - silence. Pressing keys on the keyboard entered notes, but without the usual and expected sound.

I tested muting and unmuting to no effect.

Pressing space bar for playback had a positive effect - the track played back, and I was able to enter notes and hear them after that.

Tested running the cart again, with the same resulting silence.

I'm running Windows 10 and Pico-8 0.2.4.