Television audio is transmitted over FM, which is subject to
https://en.wikipedia.org/wiki/Capture_effect
So it is easy to take over the audio channel of a broadcast.
(I've done it and can tell you so because the statue of limitations is expired many times over)
Assuming you're using a real TV transmitter and not just an FM transmitter tuned up on a TV channel you will wreck the picture, blow out the synchronization, etc.
You'd need to have 10x or more the power to take over the video signal.
(The thought of using a specialized FM transmitter to capture the audio seems like fun but boy do I feel like I have a lot to lose these days, including my ham radio license.)
