Having owned a (RevA) MBA and now 13" April 2010 MBP, I would say that the quality of the speaker is only a small part of the equation.
On the MBP the sound has far more mass to vibrate through/against. So inevitably the sound ends up richer. So even if technically the speaker on the MBA has been improved I would be surprised if it's superior for music playback. Haven't heard one yet tho and I am keen to do so.
Anyway, one thing that helps with either machine is to use VLC, the volume overdrive feature works well.