In my experience, this is simply not true. Every Blu-Ray I own that was upgraded from standard DVD has significantly improved sound as well. Assuming a quality sound-system, the difference is pretty dramatic.

And it's not simply a "better picture", that makes it sound so....inconsequential. It's a dramatic difference (some movies more than others) that for any movie-buff, IMO, is a must in terms of upgrade. Everyone I know, including myself, who's made the jump to Blu-Ray had the near exact same sentiments as you've expressed...."just a little pic upgrade, what's the big deal?"...and after a short period of Blu-Ray watching, to a man they would never go back and can't believe they waited so long to upgrade. It's very similar to those who fight the upgrade to HD..."I really don't see the need, there's not much of a difference"....until they have a proper HD set-up for a period of time, and their tunes quickly change. I'm not saying it's a must for everybody, I'm just saying if one is into movies/TV-shows, it's well, well worth it.