If you have navigation then you need to check the volume of the navi instructions.
When the navi gives instructions the audio volume of the left speaker (in LHD cars) is reduced, this has as a (very irritating) side effect that the sound moves from left to right.
If you turn the navigation volume down to zero, the left speaker does not reduce in volume.
However if you leave it just one click above minimum the left speaker volume is reduced, BUT you cannot hear the navi instruction because the volume is too low.
All you notice is that the sound moves from left to right and then back again.