G1000: Is it possible to balance the radio volume for pilots and passengers?
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 :-)