I regularly fly a DA42 4-seater equipped with a G1000 system. Sometimes my passengers in the rear seats complain that the radio volume is too loud for them but it is OK for me (intercom is just right for everybody). Reason probably is that I am using a very different headset than them which I assume has a higher impedance and thus requires more electrical power for the same audio volume. If I turned down the COM volume it would be too quiet for me.
I wonder if there is any trick to adjusting/balancing the volume in such a setup.
You can't control the radio volume separately for the passengers but you can control the intercom volume separately. If the passengers' headsets have individual volume controls you can ask them to turn down the volume and you can then compensate by turning up the passenger volume for the intercom. That way the overall intercom volume will be about the same but the radio/ATC volume will be lowered.
If all else fails, you could turn off the radio completely for your passengers :-)
Intercom Volume Control is the rotary knob on the bottom of the Audio Panel. The small knob controls the pilot ICS volume, while the large knob controls the copilot/passenger ICS volume. Just be sure to be in VOL mode (VOL in the lower left to be illuminated).
Volume for Coms/Navs are controlled by the COM radio volume control located on both the PFD and MFD, but (as far as I know) you cannot set that volume independently for pilot/copilot/passenger.