Audio problem
It used to be that when connecting your computer to your TV that it required VGA or DVI & a separate audio lead.
Newer TV's now support Sound between your PC and the HDMI (DVI-I or DVI-D) Socket.
In order for this to work you have to have a video card that supports SPDIF. You can purchase a cable which connects your sound card to your video card within the machine itself. On notebooks this should already be possible, or you may have to change a setting in your video driver to enable this option.
So what is happening is that your TV is detecting HD through HDMI which includes sound. This has the effect of shutting off all other sound sources i.e. your separate audio cable. So this is why you do not get sound.
To test this on your set: Play music on your PC connect your audio cable as you would have done with older tv's. Then select another input source on your TV. You should hear sound from your PC... but no picture.
If this is so on a desktop PC then you will have to buy an SPDIF cable to connect your video & sound together. On a notebook look into the drivers for the enable audio option (You may have to do this on the desktop with SPDIF as well).
There are numerous other solution which work but only until the PC is restarted then you will have to apply the solution again... which is a pain.
So Desktops - buy the SPDIF cable, Notebooks change the audio settings within your video card drivers to "audio on".