If I right click on an .aiff, .wav or .mp3 file and open with Audition while it is closed I get no audio, even though the wave form is displayed and audition says it is playing. If Audition is open already right click to open works. I'm on 10.6.4, but I am about to update.
ETA: On further testing I can not get any sound playback unless I record first.
Is it possible to post a screenshot of your Audio Hardware pane in Audition's preference window? Are you using the built-in output or an external audio device?
I had been using a usb soundcard and even though I set it back to built in output and restarted the computer it didn't work. I held down the shift key on startup and that seems to have fixed it. If it happens again I'll take a screen shot.