finally got rid of all beeps thanks to this one simple fix. So it seems that the PC beep takes no volume orders from alsa
but does listen to a mute command.
I found you can also run

$ amixer set “PC Beep” off

to the same effect. Obviously replacing “off” with “on” switches it back on, so you can put the two commands in a script/alias/keyboard shortcut or whatever and if you find you actually need this ear-piercing, nerve-jangling, insomniating but arguably-vaguely-useful beep, you can switch it on and off quite easily.