It made bubbles of, per wikipedia,

polyvinyl acetate dissolved in acetone, with ethyl acetate plastic fortifiers added. The acetone evaporated upon bubble inflation, leaving behind a solidified plastic film.

Which looked like,

↓↓ VOLUME WARNING! LOUD DISTORTED AUDIO!! ↓↓

And here’s a 1970’s commercial. Especially headphone users, would recommend starting low and easing into it.