I got a Mac Air a few months ago, and I found the speakers to be barely audible for watching videos or music, however, I stumbled upon the program "Boom", and it has solved the problem perfectly. It boosts the sound quite a bit, and also has an equalizer which is located up on the bar along with the slider for sound control.
I have nothing to do with the program itself(wish I did), but for both my wife and I our Airs now have all the volume we had initially hoped for.
Although it leaves me asking why Apple doesn't boost the volume in the first place.