The MMBASIC interpreter on the PicoMiteVGA supports a couple of audio output methods as standard.

I am currently breadboarding the various options to try and get a nice version for this project.

The VGA-to-HDMI converter that we have used in this project also supports an audio input (possibly stereo), that it incorporates into the HDMI and sends to the monitor.

If we can get a sensible connection to it, we then have a choice of building either on-board amplifier and speaker (like a ZX Spectrum or BBC Micro), or TV/monitor sound out (like the VIC20 or Commodore 64).

Next update when I get a satisfactory solution.