A recent posting of mine here was about the sound of our Earth, sped up 10,000 times. The actual largest terrestrial sound possible. Well, this is a posting about the smallest terrestrial sound possible; the “sound” made by molecules. This post writes about this post from World Science magazine, which starts off by saying:
Physicists say they’ve recorded tiny vibrations of individual molecules, that could be called sounds “depending on how you define sound” and put them in audible form.
You can hear the sound, here. But I doubt it actually “sounds” like that.