AFAIK all the "stop that crappy sound" proggies work in a similar way:

On shutdown they mute the Mac.
After power-on jingle is (not)played at this volume.
During boot volume is reseted to the last value the user had set before the last shutdown.

So even if there was an MorphOS proggie doing the same, you would allways need to do a proper shutdown instead of the Amiga-typical plug-pulling.

Another solution would be to set the Macs volume to 0 and than use an USB-Audio-dongle supported by Poseidon....