The real answer is that you need to check that the laptop supports HDMI audio out.
The previous poster is correct that HDMI can carry both sound and video but the output device, in this case a laptop, has to support HDMI audio.
If you want to buy a laptop that doesn't support HDMI audio then you can use a cheap lead (less than �5) to go from the laptop headphone/line out jack to the audio inputs associated with that HDMI socket on your TV.
Bottom line is that you need to check, don't just assume HDMI audio will work before spending your money.