The above posts mention good points that should be checked, however...
If you're calling this handler from a client button, that means it's a new thread firing on the call object, and probably doesn't have control of the call.
You should try adding a "Set Call State" toolstep to the handler, before you play the audio. The only parameter you need to change is "Claim control of the call", set that to true. This will give the new thread permission/control to act on the call. You should then hear the audio file being played to that call, assuming everything else is in order.