Can't add new keyboard shortcut

Hope someone can help, I'm trying to add a keyboard shortcut to open the hardware preferences, because I frequently swap between headphones and external speakers and it's a pain to have to keep going in manually. (I'm on a mac laptop.)

Looking online it seems the shortcut Command Opt ; should do it, but this doesn't work for me.

Trying to set up that shortcut manually, promisingly I seem to be able to add it to the keyboard shortcuts, but when I click OK and try it, nothing happens, and it's not there when I look again.

Curiously I have a couple of shortcuts set up for keys 1 and 2 on my keyboard for 'cut' and 'autoheal', which work, but I can't see them on the keyboard shortcuts display, even though I know they are working. (I set them up ages ago, so can't now remember how I did it...)

Would be so grateful for any help.