I had the same issue but found out how to fix it. Plug in the cable and make sure your TV is ON and receiving the signal. Then, leaving the cable connected, go to your Sound settings in system preferences. Click the Output tab, and click on the TV instead of Internal Speakers. Sound should now come through the TV. When I first got it sound would not come through the TV so I messed with the settings and found out that was all you had to do.
Hope this works for you!