To find out which app grabbed your key use the debug keysym XF86LogGrabInfo. Use xdotool to press keys + XF86LogGrabInfo at the same time, see falconer's post here ; In a terminal run
KEY=XF86AudioPlay;\ xdotool keydown ${KEY}; \ xdotool key XF86LogGrabInfo; \ xdotool keyup ${KEY}; Then check for output in tail /var/log/Xorg.0.log.
