You can get a very good laptop for under $600, but in the 15" models, the speakers will produce little or no bass in any model I've ever seen. Depending on your needs, you can get excellent sound quality in $20 headphones, or you could plug the earphone jack to a home stereo or home theater system.
I plan on buying an Asus laptop from Best Buy but I am unsure of what model. They all seem very similar. My main concern is the speakers, I like to listen to music and watch movies a lot so the sound quality is important. I also want a fast processor and depending on which one I choose I'll upgrade the RAM to 8 GB. At least 15 in screen. I want one for under $600. If you look on the Best Buy site there are many to choose from.