Hi all.
I spent some time this morning coming up with a better approximation based on plain observation as there's little documentation here.
I submitted a PR for it, so hopefully this should be improved shortly.
If anyone wants to take a stab at testing it, and is comfortable with compiling code (or using @meleu's script to install test branches) this is my dev branch, based on 2.6.3.
https://github.com/pjft/EmulationStation/tree/OMX-Volume
You may also download a binary for the Pi 3 here:
https://github.com/pjft/EmulationStation/releases/download/0.1-OMX/emulationstation
Place it somewhere on your pi, exit the main ES, run chmod +x emulationstation and then ./emulationstation and see how it performs at the different volume levels, especially compared to VLC.
Would love to hear your impressions. Be mindful that different videos have different volumes, as well as different hardware systems (i.e. emulators), but hopefully this performs better.
Thanks.