No sound on tv when connected to laptop via HDMI lead

Hi I hope someone may be able to help me. I have just bought a Studio 1745 and when I connect it to my TV with a HDMI lead I get a great picture but no sound. I am using Windows 7, my video card is 512 MB ATI Mobility Radeon HD 4570. I have been searching the net and it seems to be a common problem but as of yet I have not found a solution. My TV is LG 500 LCD. I have not currently got a mic cable to try that but that may be an option to try the two cables. Maybe the video card I have supports picture but not sound throught he HD cable? Any ideas will be much appreciated.