You shouldn't need an extra cable.
1)Turn on the laptop, with no HDMI cable plugged in.
2) Ensure Sound is set to "Speakers/Headphones"
3) Turn on TV and plug HDMI cable into TV
4) Turn TV onto the appropriate HDMI channel
5) ONLY THEN, PLUG IN THE HDMI cable to the laptop.
6) You may need to toggle "Function F8" to get the image on the TV.
7) You should now have sound and image through the TV.