If music is really important to you, you should look into getting a set of external speakers or headphones/earphones.
On my 2008 MBA, it would sound a bit muffled if I was typing while music was playing since the speaker was directly under the keyboard. Not sure about the new ones, whether they've changed the location or not.
Or just buy a Mini Boombox from Logitech :
Needs to be decent on the notebook itself!! When you pay the premium for it to be light and thin, you don't want to be carrying other bits and bobs around with you to enhance it!
I own both 13"s and you can't expect to seriously listen to music on any of them. Speakers suck on both. The Logitech Mini Boombox rocks, however.