Soundlessness is the noun: the state of there being no sound at all. It is rare and heavily literary — much rarer than 'silence', which is the ordinary word for the same idea and the one you should normally use. So why does the word exist? Because 'silence' has grown extra meanings — a person can keep silence, a government can be met with silence, silence can be a choice or a refusal. 'Soundlessness' has none of that. It is flatly physical: no sound waves, nothing to hear. A writer picks it when they want the bare fact of no sound, stripped of any human meaning, or when 'silence' would sound too dramatic. You will meet it in descriptions of deep snow, of space, of a place after a machine stops. It is uncountable and takes no article: you say 'the soundlessness of the room', not 'a soundlessness'.

無聲的狀態、寂然無聲。這個字很少用，一般直接說 silence（寂靜）就好。