Lost audio
 

Can't think how but lost all sound; although output meters are bouncing around just as I would have expected?

I hate it when that happens :)
even when working with sound environments all day
sometimes I have to double check my mixer settings and ensure nothing is plugged or unplugged to block the audible sound.

I can't think what else to check

Run alsamixer from CLI, see if nothing is muted.
Verify or replace your cables and hardware.
Boot from a live CD, and see if sound works.

Kind regards

Or just try a quick reboot that may restore your settings if they got confused.