Following Vince's ( Vincent Meyer) excellent idea and constant support, after a few tries I ended up with the excellent sound setup for my situation.
Now, I can adjust SEPARATELY X-plane aircraft sounds and ATC volume, not to mention that the quality also improved a lot which in case case is VERY important.
Thank you, Vince.

Attached picture explains pretty much everything.
I am posting this just in case someone else would be interested.

Cheers, AJ