From what I understand, trying to change the volume whilst it's giving directions only allows you to turn the voice volume down to 1, not off
To do that requires the settings area the OP has already investigated
It sounds like the garage messed it up so should be approached to fix it
Just out of interest does the normal audio mute when it's trying to give instructions.
Clutching at straws .... the SatNav only comes out of the driver's side, is the audio balanced to both sides