I had that happen on my S70. The car was under warranty, but I decided to get aftermarket speakers for the back. I got Infinity something or other. They are smaller than originals, so I had to make a bracket to adapt them. I used the original speakers. I don't rmember exactly, but I had to cut the part that mounts to the rear shelf from the originals and then make the smaller speaker mount to that. In hindsight, I should have gone for the warranty replacement. The Infinity speakers are much better quality to be sure, but they ended up not sounding THAT much better than the original equipment. I think the reason is flawed mounting. I still have open gaps between the speaker and the mount, so the bass is not fantastic. Highs are good, though. I paid something like $130 for the pair of Infinitys. The moral of the story is that if you can get your hands on cheap or free original equipment, go for that, otherwise, $130 and some creative installation techniques may be necessary. The sound will be comparable either way. Even though the originals are crap paper cones, they do the job well since they are well-mated to the car. My Infinities have not failed yet, though - no flatulence sounds at all. I may re-visit the mounting I did and may improve the bass response.

Good luck,
Vladimir