[thro1]

For the first not so easy - what about soundproofing ? (For sound to be able to vibrate through an object, the object itself must be able to vibrate. This is why extremely dense materials make for great soundproofing as they are too thick to vibrate. Mass is a fairly straightforward soundproofing principle — the greater an object’s mass, the harder it is for sound to shake the object or push through.. - colder air is heavier, so energy (even if more ..or regardless(?) of transmission (into..?) ) to shake it shall last for shorter distance - right ?)

About the second: When air is colder, the molecules are closer together, so sound transmission should be easier. This is true of solids and liquids, but gases behave a little differently. When gases heat up, their molecules move much more quickly. This increased vibration transmits the sound more quickly than it would in colder, but more static, air. ( https://www.soundproofcow.com/sound-travels-different-materi... )

[anu7df]

That sounds a bit odd. Sound is coherent motion of the conducting medium, whereas motion of gas molecules is random. I always associated the increased speed with reduced density, and hence reduced "inertia" with increasing temperature.