Are non-verbal spells in Harry Potter silent or just quiet?

Question

After a few too many cases in the last two books where someone is said to have "muttered" a spell, I've lost track of how non-verbal spells work.

Are they absolutely silent, or just quiet?

Answer 1

Silent (if done properly).

Harry frequently looked over at his classmates in the common room or at mealtimes to see them purple in the face and straining as though they had overdosed on U-No-Poo; but he knew that they were really struggling to make spells work without saying incantations aloud.

_{^{Harry Potter and the Half-Blood Prince}}

and

Pointing his wand at nothing in particular, he gave it an upward flick and said Levicorpus! inside his head.

‘Aaaaaaaargh!’

There was a flash of light and the room was full of voices: everyone had woken up as Ron had let out a yell.

_{^{Harry Potter and the Half-Blood Prince}}