I connect my PC laptop to my plasma screen tv with a HDMI to watch videos but there is no audio from the tv?

I ran into this exact issue with my LED TV connected to my PC HDMI out. Here's what I did (windows 7) On the bottom where the little speaker/volume icon is, right-click. Choose 'playback devices'. Since you hear sound from your PC, you should see it with a green check mark next to it. If your TV is on and connected, you'll see your TV listed there too. Next click on your TV to highlight it, and mouse down, you'll see a button that says set default. Hit it, and now your TV will have the check mark. Sound will now come out of your TV. On my PC sound will only come from the TV or my PC but not both. Hope this helps. Don't forget to change back when you are done.

HDMI is digital video and digital audio. Sounds like your computer has a DVI output, which is digital video, but no audio. And no way to combine audio, since your sound card is analog. You have a couple choices. Install an actual HDMI interface in your computer which will combine both the video and audio output to go to your TV. Or, if your TV has a VGA input, use your computer's VGA monitor output, plus a separate audio cable from your sound card to your TV's VGA input.