It's been quite a while since I've messed with AD2 so go easy on me.
With a MIDI clip selected I hit the green play button on the AD2 audition player and the green light turns off but the clip doesn't play.
What am I doing wrong?
 
Thanks very much,
R

If you are using the stand alone version, check out the audio routing settings. Make sure they are going out to your appropriate audio card. That would be a place to start. Any time I am using a synth or ad2 or the like in stand alone , I need to check that.

Otherwise maybe describe how you are using it.

I believe you have to have the brown "sync tempo" button engaged as well.
 

It should play without the sync button. My guess is the Audio Engine stopped. The Green play button will still show when the audio engine has stopped in Sonar.

In stand-alone mode you might have to twiddle with your audio settings some.  I hadn't used the stand-alone before but I checked it out to try to figure out your issue and I had to set the audio up, close the program then adjust again, then reopen, then everything worked fine.
 
And it played with the sync tempo unchecked or checked, but at different tempos.  Not sure what it was syncing to in stand-alone mode.
 
Let us know if you're having this issue using AD2 in Sonar instead of stand-alone.