Blow speakers.

depends on what sound quality you're looking for. i'm happy with stock, but an audiophile might want an improvement, possibly an amplifier as well

 

To confirm all 4 speakers are blown hook up some wires to each speaker and tap those two wires to a 1.5v or 9v battery and if you hear a clicking sound the speaker are not the problem. But if it is the speakers:

You could buy 4 speakers online for less than $100 total that will sound better than the original ones. But don't get suckered into super inexpensive car stereo speakers, they're a total scam that is paying for car stereo over-priced rip-off culture not sound quality.

 

i have never heard of all four speakers going bad. the problem is likely somewhere else, including abuse by the prior owner

 

Interested in how this worked out. if they were JBL speakers I would be shocked if all were blown. I had JBL's, and again not factory installed as they are a bit less quailty, but abused the hell out of them and they never failed. I had a system with JBL's that had people running and screaming from their homes thinking the band was going by live it was so loud and clear.
I have the JBL system in my 2007 and while not the best I have heard, it seems decent enough. I am an audiophile and need to get my Iasca disc in it to tune it up. But in the two weeks I have had the car, it is so much better than GM's Bose systems, which quite frankly are pathetic at best I am fairly happy with the JBL system for the moment. Shutting off the rear speakers, the JBL's have a decent soundstage. I can hear a good mix of the music and vocals across the dash. I started to incorporate a little of the rear speakers and it just created a confusing mess. So they are useless.