I downloaded it too and thought it looked and sounded better but that is obvious since it was in HD unlike on my craptop
I've got the HD (720p) version of both trailers on my PS3. Again, I don't think it's a necessarily better sound quality, but the specific effects are different. Comparing them between two different platforms may not make those differences all too noticeable, especially if they have different sound capabilities (computer speaker vs. TV speaker) from one another. The picture quality didn't change, at all, for me, but again I have both on my PS3, so I've got that direct comparison without differences due to platform differences corrupting the observations. Just to stress my point, what strikes me as odd is that specific, in-game actions (drawing a sword, hitting enemies, swinging in the air, etc.) produce different sound effects in the two trailers and I don't know what should be expected in the final game on any platform.