I remember as a kid going to a friend's house, and his dad had converted their garage into a music studio of sorts. It was mostly a jam space for rock and roll. But the thing I recall most is that the walls were covered with egg cartons. They told me it was for sound-proofing. Who was I to question it. For years I thought you could do sound treatment with egg cartons. Luckily I never spent time or effort trying to acquire cartons and do this myself! I might have thought I could build a sound-proof closet-come-vocal booth for doing auditions and other recording for voice over jobs...or worse. I could have decided I needed to cover every square inch of a garage! Shudder.
Here's an article discussing the efficacy of egg-carton sound-proofing: orignal article