Inaudible

Some high-pitched sounds are inaudible to adults, but can be heard by children and teenagers.

Tom's screams were inaudible.

That voice is inaudible.

Some multilingual people think that English is somewhat inaudible.

There could be an invisible, inaudible, intangible dragon next to you right now.

Dogs can hear this whistle, but for people it is inaudible.

At first his voice was almost inaudible; but soon the spirit mastered the body, and his hollow but distinct tones gained a supernatural strength.