A very common musical myth is that egg cartons can be used to soundproof a room.
I'm pretty sure that they must be confusing egg cartons with sound treatment foam, but I've never known anyone crazy enough to glue egg cartons to all their walls.
Funny you should bring this up, because at school one of the music rooms did have the walls covered in egg cartons, but I don't know if they were filled with something else (e.g. cotton wool, to pick something at random).
A very common musical myth is that egg cartons can be used to soundproof a room.
I've never heard this one...are you sure they aren't talking about egg crate foam?
Nope, unfortunately I'm talking about egg cartons. And I'm really jack of people telling me that they work, when I know it's highly unlikely that they do.