What moves, and what does not
Watch one particle in the scene. It sways forward and back, always around the same spot. It never travels down the tube. Yet something clearly does travel: a band where the particles crowd together, followed by a band where they spread out, sweeping from the speaker to the far end.
That travelling pattern is a sound wave. Where particles crowd, the pressure is a little higher than usual: a compression. Where they spread, it is a little lower: a rarefaction. Each particle passes the squeeze on to its neighbour and returns to rest. The energy moves; the air stays.
Pitch is spacing
Turn the pitch up and the compressions come closer together. Turn it down and they spread apart. The distance from one compression to the next is the wavelength, and the number of them that pass each second is the frequency, measured in hertz. Higher frequency means shorter wavelength, and we hear it as a higher note.
Try it: choose Low, then High, and switch on the Pressure view. Count the peaks along the tube.
Human hearing runs from about twenty hertz to twenty thousand, though the top end fades with age. A bass drum sits near the bottom of that range; a whistle near the top.
Loud is far, not fast
Loudness is a different dial. Turn it up and every particle swings further from its resting place, so the compressions squeeze harder and the rarefactions spread wider. The wavelength stays exactly where it was. That swing is the wave’s amplitude. A loud note and a quiet note of the same pitch have the same shape; one is simply taller.
Speed depends on the stuff
Sound needs something to travel through, and it travels at a speed set by that something. In air at room temperature it covers about 343 metres each second. In water it is roughly four times faster, and in steel about seventeen times. Stiffer, denser materials pass the squeeze along more quickly.
Switch the medium and watch the wavelength change even though the pitch has not. Speed equals frequency times wavelength, so a faster medium stretches each wave out.
Where there is nothing to squeeze, there is no sound. Space is silent not because it is far away but because it is empty.
Hearing it
Your eardrum is a tiny membrane that the compressions push and the rarefactions pull, a few hundred to a few thousand times a second. Bones in the middle ear pass that wobble to a fluid-filled coil in the inner ear, where hair-like cells turn it into nerve signals. The whole chain starts with the same thing you see in the tube: a pattern of pressure, travelling.