Muting sounds of other sources when there is an outgoing or incoming call

[Angelo V]

Is there a way to mute all other sounds (example youtube or windows media player) to mute all other channels while a call or ringing is in progress?

 

[SKDamon]

That's a windows setting. Go to Control Panel -> Sounds -> Communication, there cou can set what Windows will do when it detects communications going on.
(I didn't test it, but since 3CX Client registers as a telephony app, I don't see why it should not work)