DISPLAY VOLUME CONTROL PROBLEM

Hi,
I developed videoplayer application. In that i  developed the display volume control on navipane but it works only on the pause state of the clip, it's not working on clip play time at the time i press navi key total application also closed. I want this application same like real one player and also i want that mute and unmute symbols on my navi pane.pls anyone know send answers as soon as possible.