Yes, I felt the same way too. I still have my 2010 13" MBA so I compared them and the MBA 2010 speakers sounds better, more direct, and have a bit more bass. The 2011 13" MBA sounds sharp and the bass is not as good.
I believe the reason is due to the extra backlit keyboard. With the 2010 MBAs the stereo speakers are directly below the keyboard without any cover over it. But, in the 2011 MBA, the speakers are actually below the backlit keyboard film which somewhat shielded/covered the sound from coming out of the keyboard gaps.
Having both side-to-side playing music, I found the 2010 MBA much more enjoyable to listen to and I can listen to it all day. But on the 2011 MBA, the sound and bass are sharp and I do not enjoy listening to it over a short period of time. I guess having the backlit keyboard, you have to trade-up on speaker sound, but I don't listen to music or watch movies on MBA all day (I'd use headphones). I'd leave the movie watching and music listening to the home theater or my iMac with external speakers.
PS: The MacBook Pro 13" doesn't sound as bad probably because the speakers are behind the thin metal strip on the side of the keyboard so maybe the reflects the sound better vs covered by the backlit keyboard film.
*If someone willing to tear the MBA 2011 keyboard apart and play music, please let us know if it sounds better that way.