The tooth fillings acting as a radio receiver is based on an urban legend, one of them reporting it as true is actress Lucille Ball.
Per the title, hi-fi or high fidelity was the term used by record enthusiasts to refer to high-quality reproduction of sound that had minimal amounts of noise and distortion and was generally applied to record players which possessed stereophonic sound.
Mary Ann loves to listen to soap operas on the radio. She mentions three programs: In X Marks the Spot she mentions Young Dr. Mallone; in Hi-Fi Gilligan she mentions Blaze of Noon; and in The Postman Cometh she mentions Old Doctor Young.