GMC don't have sound or back, passenger side don't have sound.
1)Check the console to see if the setting is right.
After you put those settings, turn off the car, open and close the door to let the unit to fully turn off and turn on the car after 10 mins.
2)Use an external speaker to connect to the RCA to see if the unit has the sound.
This could check if it's the radio issue or the setting issue.
3)Go to advanced setting-1688,1680- find the audio fador to change it-save and reboot
4)check the system info to see if there needs the update.