UPDATE: (I assume I can’t edit my previous post?)
I tried clicking lots of things to see if I could unlock the Master control. Eventually I discovered the solo control in the right-click menu of the audio file in the project panel. Clicking this re-enabled the audio.
Oddly, clicking the small headphone solo icon, as I had earlier, did not work. If it happens again I’ll try the keyboard shortcut and see if that works.
Still, I’m left wondering why undoing an action should have any effect on the audio master.