I was fumbling with the screenbar mixer as well as the sound preferences for every unit and driver there is - to no avail. The sound volume remains constantly low.

What is it I am missing?

With the default driver (AppleI2S: Hifi 16-bit stereo++, Volume slider at max, monitor & gain sliders at min, debug level & default anti-click set to min, volume scaling=full volume, all other options greyed out) I have sound at audible levels (not very loud though) when the volume screenbar slider is set to max.

I have to 'move' the the volume screenbar slider in order to enable audio though, even if it is initially set to full.

Haven't tried setting audio to max in OSX yet to see if this makes a difference.
Since OSX volume affects the boot-chime volume on powering up, it must be stored in NVRam and read at power-on. I would guess MorphOS doesn't overwrite this value until the volume slider is moved.

To pick up this old one, my problem (constantly low sound volume regardless of any setting anywhere) remains even after resetting NVRAM via four fingers salute or open firmware. Plus, headphones on line-out won't work at all.

Well, I actually can live with all that, but...