As one already said, you need space behind the speakers (air) to make them sound better. Supposedly the MBP speakers are to be better than the PB speakers. Are they??? I think not.
What the heck, buy a nice set of head phones if you want sound. On the other hand, there are laptops with pretty good sound, it's just that they are packed into a larger formfactor...
My friend just got the latest rev. MBP and the speakers on my 5 year old IBM Thinkpad sound better than his...
It seems, that APPLE STILL has not been able to solve many quality issues. According to a recent survey 30 (THIRTY!!!) % of the owners of new MBs or MBPs have problems, and that is not counting those who think whatever issue they might have is not really one....
This is way to high a percentage of production output with problems. It actually proves unprofessionality on the part of APPLE QC.