softness - a sound property that is free from loudness or stridency

music - an artistic form of auditory communication incorporating instrumental or vocal tones in a structured and continuous manner

decrease, diminish, lessen, fall - decrease in size, extent, or range; "The amount of homework decreased towards the end of the semester"; "The cabin pressure fell dramatically"; "her weight fall to under a hundred pounds"; "his voice fell to a whisper"

decreasing - music