Option A - 'Invisible' is used for something that can't be seen. Hence, option A is incorrect.
Option B - 'Irreparable' is used for something that can't be mended or repaired. Hence, option B is incorrect.
Option D - 'Unheard' is used for a person who is not listened to. Hence, option D is incorrect.
Option C - 'Inaudible' is used for a sound that can be barely/ or can't be heard. Hence, option C is the correct answer.
A sound that cannot be heard - Inaudible.