Hey folks,
Tweaking the Retropie sound settings (accessible via whatever button you’ve got mapped to Start) got the software volume control working properly for me. Once I’d changed the audio card to PCM and the OMX PLAYER AUDIO DEVICE to ALSA, adjusting the volume slider worked properly (and it also got rid of the volume control errors).
Let me know if this solution works for others, and if it does I’ll add it to the tutorial :)
Hel x