I've switched from RecalBox to a new RetroPie build, and recording is working again with audio.

The audio driver in RetroArch is set to alsathread (not pulse).
When I tried alsathread in RecalBox I got no sound, i only tried this once though.

The solution for this issue on RecalBox may be to switch from pulse to alsa or alsathread as the audio driver.

Thanks! Solved!