I've just tried to audition some loops and I can't here anything. I know the loop is playing because the button is blue and I know my audio is working because I can the track I'm working on plays fine. I just get nothing when I click on a loop. 
 
It was working yesterday.
 
Any ideas anyone?

Just opened a new project and the loop preview works so it seems to be specific to the track I'm working on...........

Okay. I figured it out. I'd deleted the preview bus so I just routed the the audio preview to the master bus..............
 
Move along folks............nothing to see here :)