They could have used 320kbps encoding and it would sound pretty much the same, unless someone who plays the game is an anal retentive audiophile.
I thought it was common knowledge 30-50GB games are going to be the standard for this generation of gaming anyways? Moar graphical/audio fidelity = larger games, and I don't think dev's are going to make compromises just to save space for people who purchase digitally.