Now, extending this, can you figure out a way for GarageBand to produce "anti-sound" in response to the ambient noise, so that all you hear is silence? That would be a very, very good trick.

Until then, I'll keep my bulky Bose QCs on....

I believe for this to work you would need a mic right next to each ear, then have your Mac invert the phase of the sound coming into each mic within as few samples as possible, then output the sound back to your headphones. In other words, I think your best bet would be to buy some noise canceling headphones =).

Oops, I guess I mean you'll need to keep your bulky Bose QCs on.