I know it is a long word other than humming. There is a name for the action of talking with your mouth closed in movies.
The answer depends very much on the circumstances.
If you hear the character's voice, but none of the other characters do, it's a voiceover.
If multiple characters are conversing without speaking aloud, it's telepathy.
If the character is intentionally throwing his voice (multiple characters can hear him, but he isn't moving his lips), it's ventriloquism.
If the character is speaking softly without moving his lips (for the benefit of only one or two other characters standing close by), it's subvocalizing.