SugarCube 2: combining click and back macros?

I am using the <<click>> macro to fire a sound effect when the player clicks on a button, but I'm having difficulty getting this to work with the button I've set up to undo choices. It works fine progressing forward:

<<click "Continue" "Start3">><<playsound $buttonGo>><</click>>

But it breaks when I try to undo:

<<click "⇦ Undo" "back">><<playsound $buttonBack>><</click>>

It either looks for a passage titled back when I use quotes, or throws up an error if I don't. I've looked at the other macros available and can't seem to figure out the best way to incorporate this.

Any ideas?