fade down
To gradually decrease in intensity, often of sound. A noun or pronoun can be used between "fade" and "down." Fade that song down, will you? I don't want it ending quite so abruptly.
fade something down
to turn down a sound. (Broadcasting.) The radio engineer faded the music down and the announcer's voice began. She faded down the music.
fade down
[for sound] to diminish. The roar of the train faded down as it passed and fled into the night. As the thunder faded down, the sun began to break through the clouds.