1. Check to make sure the audio and video cables are firmly connected.
2. Ensure that your TV is connected to an appropriate power source.
3. Try toggling the audio settings on the TV settings menu.
4. Try power cycling the TV. Unplug it for a few minutes then plug it back in.
5. Check the audio and video settings on both the TV and the device you are connecting (laptop, gaming console, etc)
6. Restart the streaming application you are using (Netflix, Youtube, etc)
7. If your audio is still out of sync and the TV is connected to an external sound system, try resetting the sound system.