Roundabout is AC2's garage theme and Artificial Sky II is AC3's garage .
Yes they sound different in the OST compared to console versions . Almost every garage bgm from AC1-ACLR has been modified in the OST releases .
If you have winamp and Highly Experimental ( PSF1 and PSF2 players ) , it can play the home console bgms of AC2 and AC3 garage PSF2 files I ripped which can be found by google ( I sent my to llin site I believe ) . If you don't have that but you have FL Studio , you can download AC2's garage FL Zip from my youtube video using FL Studio . Not 100 % accurate but still sounds same imo .
FL Studio Roundabout Youtube :
http://www.youtube.com/watch?v=0zxcYdEQqOY&feature=channel_page
Note : The video is not the same as the one in description since it got updated .