Usually, people are asking as if there is just "the" amplitude in sound waves in air.

The loudness perception of a sound is determined by the amplitude of the sound waves −

the higher the amplitude, the louder the sound.

Which amplitude of sound (sound amplitude) do you mean?

There are:

amplitude of particle displacement ξ, or displacement amplitude

amplitude of sound pressure p or pressure amplitude

amplitude of sound particle velocity v, or particle velocity amplitude

amplitude of pressure gradient Δ p, or pressure gradient amplitude.

Furthermore, think of the amplitude of the oscillation of a string.

The maximum magnitude of the deflection of a wave is called amplitude.

Look at link: "What is an amplitude?"

There is not only "one" amplitude.

There is an amplitude of particle displacement ξ, or displacement amplitude,

an amplitude of sound pressure p or pressure amplitude,

an amplitude of sound particle velocity v, or particle velocity amplitude,

an amplitude of pressure gradient Δ p, or pressure gradient amplitude.

If the "sound" inceases, the "amplitude" also increases.

The term for maximum displacement is the amplitude of the wave.

amplitude increases (louder the sound is, bigger is the amplitude)