It's an OS Booting sound, not a startup-sound and therefore it's under the control of the OS you are booting, you'd have to configure MOS TO TURN IT OFF.
There is no option in MorphOS to turn it off.
Install startup sound prefs panel (startupsound.prefpane linked by Jorkany) to OSX where you turn it off. Then you can erase OSX and startup sound is gone forever.
