I seem to remember the sound was about the same. (it's been a while since I've played DVDs or CDs in my PS2). What with digital-out and all, there shouldn't be any noticable difference.
The video, on the other hand... I was severely disappointed. The image was waaay too dark, almost unwatchably dark. Very soft, too.. softer even than my old Panasonic A110 that finally died last year after seven years of faithful service. I replaced it with a JVC, which prompty went back to the store because it had problems actually playing movies, which isn't what I want in a DVD player. I replaced that with a Sony uh.. NS575, I think it is. That was only about $80, and it's far better than any of the other ones I owned. The difference between it and the PS2 is night and day. Literally.. the PS2 is night because of how frikkin' dark it is, the 575 is day.. hehe. Much sharper image, brighter colors, and it's pro-scan capable.
Bear in mind that my PS2 is a launch unit, and quite old. While age itself doesn't really play a factor, they may have improved the playback on later generations of the machine. A brand-new one bought today might be a lot better than mine.
All in all, though, even the PS2's strongest supporters agree that, as a DVD player, it leaves a LOT to be desired.