Mine does the same even after I had the rear deck replaced because of it. Others disagree, but the sound system is just a weak point on the car unfortunately.
Do a quick search, there have been a few very detailed threads about people who have made serious mods to the sound system with good results.
I did manage to blow out my rear speaker in my Air. The service folks said they hadn't seen that happen before. They replaced it under warranty for me.
Having said that, many more people have had issues with their rear parcel shelf, which does sound remarkably like a blown speaker. Either way, take it in and they can tell you for sure.