When using the Lip Sync feature, a character's lips will open more widely when the underlying audio file is recorded at a higher volume. This allows you for a "whisper effect" when a character talks. If the narration is recorded at a low mic level, the lips will open less than they otherwise would.
Audacity is a program many creators use to record narration - click here to learn how to change the recording levels.
Note: adjusting the volume of an audio file in Vyond Studio will not affect lip movements. The adjustment must be completed prior to importing.