By default the commodity Mixer is installed in wbstartup. But what does it do?
Lookin at its interface, everything is diabled, and it says no device installed.

Another sound related thing. If I enable the module "Volume" from the top menu, I will get a small icon for setting the volume. But this doesnt do anything to the voulme. If I mute it or turn it to max its still the same output to the speakers.

Mac mini does not have HW volume control. Hence it is not currently possible adjust volume from Mixer or volume control but this should change in the next MorphOS release.

IIRC there is no HW volume option on the Mac Mini, so it has to be done through SW, which isn't implemented in MorphOS 2.4. Could be wrong though...