do you have the lcd information center deal like this?
if so, then i would imagine there's a setting for it within the settings button. probably somewhere near where you'd adjust the clock? i'm out of town so i can't check mine.
otherwise maybe in the diag menu? IIRC i think you turn the radio off, turn the car off, turn the car back to the ACC (or maybe ON, but don't start it) position then rotate the volume knob 20 "clicks" to the left or right. can't remember where i saw how to get to it, could probably search for "pathfinder diagnostic menu radio knob" and find it.