Definitions first. I will define sound as pressure waves in a medium. That is, sound is mechanical oscillations in an elastic medium. The medium, such as air or water or steel, is pushed in one direction by pressure, then falls back elastically in the other direction before being pushed again. The frequency at which it oscillates determines the pitch of the sound. When middle C is played on a piano, for instance, its string vibrates 261.626 times per second, pushing the air at that frequency and jiggling your earbones at the same pitch. In so-called scientific pitch notation, middle C is set at 256 Hz, which is much more convenient because it makes all Cs round numbers in binary arithmetic. We don’t need to worry about the difference. And for the record, yes, there is sound even when no one hears it.
An interesting note about sound is that it requires a certain minimum space in which to form. If a sound wave is ten meters long, it can’t form in a five meter room, so you won’t hear it. This gives us a clue about our original questions about the lowest possible sounds. If there is a limit, it will depend on whether or not the universe is finite in size. If it is then yes, there is a lowest possible sound frequency with a wavelength equal to the size of the universe.
The lowest note discovered so far was found ten years ago by astronomers using the Chandra X-ray Observatory, a space-based telescope. They have observed ripples in intergalactic gas clouds surrounding the cluster of galaxies they call Perseus. They’re definitely pressure waves, alternating higher and lower pressure. If you had a big enough ear you’d be able to hear a B-flat fifty-seven octaves below middle C. That doesn’t sound like much until you realize that for every octave you go down, you double the wavelength. When you’re doubling like that it doesn’t take long to get really big. The frequency of this note isn’t measured in Hz, or cycles per second. It’s measured in cycles per ten million years, so you might get a little restless waiting for the concert to end.
Now that’s infrasound.