Question

My grandfather spoke so quietly that his words were almost audible. Audibility. Inaudible. Inaudable.

Answer 1

My grandfather spoke so quietly that his words were almost inaudible.
“Inaudible” is the only word that fits because it means “not easily heard”, and quiet speaking would be more difficult to hear than louder speaking.

Answer 2

One, two, three, four, here's your answer so do not fear