Hi Mearbhall,
Welcome to Apple Support Communities. We understand that you're having some issues with dictation on your iPhone 12.
What exactly happens when your microphone picks up the TV or other sources? For instance, does Siri come up? Are you using headphones when this happens? If so, make sure the no buttons are being pressed.
From Settings > Siri & Search, are “Listen for ‘Hey Siri’” and “Press Home or Side Button for Siri” enabled? If so, test without them if possible and see if the same occurs. You can learn more about these features from Ask Siri, Dictation & Privacy.
Do you see a blue microphone icon in your status bar? If so, that may be Voice Control. Check out Use Voice Control on your iPhone, iPad, or iPod touch.
Take care.