Wow, thanks for the great advice. It's definitely a bit more complicated than I imagined :o
It'd be a fun project, but I think time limits me to just putting on a PhatDac, and being very grateful!
The sound issue is my own fault, I thought I was clever by getting cheap monitors and installing Kano OS on my own for my two boys. The monitors have no audio output so I made a set of speakers and mini-amp with volume-pot for each Pi as a little intro-to-electronics project.
Then I set up a Zero with RetroPie and a Zero4u to introduce them to games but without audio (HDMI audio is pointless with no monitor speakers) it isn't the same. PhatDac works but RetroPie bugs audio output to constant 100%. The little Anker speaker I have available doesn't do volume down (?!) so I thought I could rig the bonnet as a simple hardware-only cure.
I'd need a custom box/case/thingy to stop little fingers breaking buttons, I'd like to create a console with them but perhaps I'll wait until someone has blazed the trail and worked out all the gremlins.