I doubt this is true. See those holes near the bottom / on the back of your computer speakers? Play some music and shove your thumb in them while it's playing. You'll hear the difference.

Air needs to move for sound to exist, even in the direction away from your ear. If stuff's in the way, the sound's going to be muffled.

There is a huge difference. I have a pair of Grados with hard, open foam pads. They came with soft, closed foam. The closed foam covers the speaker and you get muffled sound. I am not an avid audiophile but I have mixed music for my church some years in the past.