I just found your post, yea I've noticed the same problem. All my AC/DC stuff sounded so crisp and clear on my i4 with 5.1.1, still does.
However, on my 4S with 6, it does sound funny, muffled almost. What I really love the best about iOS 6 though is how for every single text I get, it plays the notification sound twice. One message, two tones... awesome. Seems the list just keeps growing for problems with iOS6 but Apple won't ever acknowledge them, if anything they'll just disappear in a future update (maybe).