I have recently bought a cable, its an HDMI one, works fine even has sound on TV when the sound is down on laptop. Nothing like porn on a 42" plasma!
Someone more in the know should be able to help you. Maybe try AV / HDMI channels if you have them?
My tv has an input for a cable from the headphone socket from the laptop. You need a cable where both ends look like the one that goes in the headphone socket ( like those used to connect an MP3 player to external speakers)
Also you need to look at what your PC is using as the output device for sound.
If you right click the little 'speaker' icon at the bottom right of your PC, it should bring up your choice of speaker device. You might need to 'disable' the laptop's speakers in order to 'enable' the HDMI speakers. And then do the reverse when you're done watching via the TV.
And if disabling things mean they disappear from your list of choices, right click anywhere in the box and pick the option that's something like "always show disabled devices".
Had to solve this for the missus the other week, and though I've written the above from memory, PM me if it is an issue and I can try and give you the accurate advice.