My PS3 is connected to my Digistar LCD TV by way of a HDMI cable but I only get video and no sound. HELP!!!!!!?

I tried using my original PS3 AV cables and the video and audio worked perfectly. I also tried using the HDMI for video and AV for audio but that doesn't seem to work either because I still don't get audio. I have literally watched every YouTube video and read every forum about this problem and nothing seems to work for me. I am starting to get really pissed off and need help ASAP.