Running latest Audition CC on Yosemite. Audio track is displaying, I can see the levels as I scrub but NO sound to headphones. WHY?

Hi,

I am Running latest Audition CC on a 2009 MacPro running the latest Yosemite.   I can see that  the audio Audio track is displaying, I can see the level rise and fall as I scrub but I can hear NO sound to headphones. WHY?

I have checked the MacPro's system preferences and clicked through the Audition settings but I can't see where the output to device ( headphones) might be muted.  What am I missing?

Thanks