Go into the menu and check that the audio is set to "stereo", if it is set correctly set it to something else, store it, then reset it back on stereo and store it. I suggest this procedure in case the device has developed a glitch in the audio section of its logic circuitry and this procedure will (or should) correct it, of course the other thing to try is the battery out / in procedure which resets everything. If however none of these actions corrects the fault then I am afraid a bench investigation is required.