My 2008 X-160 had 2 speaker buttons on the radio - one for the inside set and one for the outside set. If both were set to off, no sound could be heard.
ON EDIT: I don't remember in that 2008 version, but in this new radio, it's difficult to tell which set of 3 speakers are set to on.
2018 Lance 1475
2013 Nissan Frontier, aka Pretty Red Truck