Set your laptop to extend the display to the TV not mirror it. Set the 2nd display to the native resolution of the TV in graphics properties. Drag the movie playback program to the TV screen.
If you are already hearing the sound from the TV then you are OK for audio.
The order you turn on the laptop and select the TV input can affect the way they "handshake". Sometimes you have to select the TV input and then boot the laptop. Sometimes select the TV input after the laptop has booted. Depends on the TV.
You can also try a different input on the TV and a different HDMI cable.