2016 MBS 2401R radio/navigation
Makes no sense. It should work fully without the house batteries turned on. Check the equalizer to be sure the sound isn't set all to the back. Also be aware of the mute function of the on/off switch. Once turned on, if pushed again it mutes then pushed again it unmutes (useful for personal conversations, non-Bluetooth connected phone calls, or when speaking with the state trooper....).
Land of 10,000 Lakes
2016 Forester MBS 2401R
2016 FR Forester 2401R
Towing 2014 Honda CR-V