[Mr.ac]

Yes, many people have done all kinds of audio upgrades. All depends how much you want to spend and what kind of upgrade you want.

I for one just added a sub and amp. Nothing crazy just a nice 200 watt amp and 10" sub in a generic enclosed box.
New amp was $80. Sub box was $40. Amp working kit $25. Line level conveter $15. The sub was from my previous car.

I would highly recommend you look up your local car audio store and take a stroll inside.