decrescendo

An instruction to play gradually more softly.

A gradual decrease in volume or loudness of a piece of music.

The peace was ended by the sound of an approaching train, the steady exhaust beat that has its own appeal, and amid a cloud of steam a rebuilt "Scot" burst from under the bridge at the south end of the station, and roared past at high speed. A long line of lighted windows flashed past, and then, in decrescendo, the sound and fury passed northwards.

To gradually become quieter

becoming quieter gradually.