I thought the same had happend to my passenger side speakers and tweeter too... I called the dealer and some dood told me the sound system was EQ in Hall mode or something like that... I was like "really.. u think I'm stupid.. I haven't touched any EQ mode and the tweeter sounds like its dying and the speaker has no sound; they suddenly stopped working correctly!" Well I took it 2 the dealer and effectively, it was equalized as "Hall mode" or something like that, other than Rock (which is where I usually have it... or just off) and the tweeter and side speaker came back on!!!!
Try that!