It's most probably a compatibility issue.
Are you by any chance in Windows Vista? If you are, and if your TV tuner card's drivers are not Vista compatible, usually there'd be no sound from the TV application.. I know this coz' I've faced it myself.
One option is - If you do have a dual boot system, try out the TV tuner card in Win XP.
Or,
Try the application in 'Compatibility mode' as Win 2000/XP in Vista..
All this advice is considering you're running Vista

I might be totally wrong here !

Unless we know the details, there'd be no way out.