Sound playing music? Sound on phone calls? Young, blasting-music-damaged ears? Old ears like yours? Probably older ones like mine that require pretty powerful hearing aids?
That's one consideration. The other one is personal preference. When the phone signal starts losing bits (it's called "going digital"), I prefer the way it sounds on a GSM device. I have more trouble understanding bad audio on CDMA because it "goes bad" in a different way.
Then there's the question of which music. Beethoven's Fifth isn't going to sound as good or as bad on the same phone as gangsta rap.
Some of the larger corporate carrier stores have working demo phones. You decide which carrier covers the areas you need best, then you go to one of their stores and try all their phones out and decide which one sounds best, has the best "heft", looks nicest, meets the budget best, etc. (Always choose the carrier first. The best phone, if the carrier has no coveage where you need phone service, is useless.) And read the forums before you go. If "phone X" is giving everyone problems, you don't want that one even if they're giving it away free.