Hi there

Firstly we need to rule out whether the TV is at fault or not.

1. Unplug all connected equipment to the TV and only have your aerial connected (as well as power)
   
2. Turn TV on, and test a freeview channel. If sound works, skip to step 5. If there is still no sound continue on.
   
3. Ensure that you have the latest firmware releases, and perform a factory reset of your TV. Then try step 2 again.
4. If everthing still fails, and nothing is connected to the TV, then Im going to say that the TV is at fault and requires a repair from an Authorised Sony Repair Centre (or any extended warranties that you may have)
   
5. If you are getting sound from your TV, but not when your sky box is connected, go through the settings to see if there is anything out of the ordinary. All else fails, contact Sky.

Hope that helps