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In the sound menu of System Preferences, click on the "Sound Effects" tab and uncheck "Play feedback when volume is changed"

That should do the trick.
 
yes - Thanks that works - but I wonder if there's a way to find the sound file in the system and replace it with silence - or do you think it's a firmware thingy?

Ssytem Prefs > Sound > Sound Effects > Uncheck "Play feedback when volume is changed".
 
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