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 to create a "whisper effect." If the narration is recorded at a low mic level, the lips will open less than they otherwise would.
If you're using Audacity to record your narration, here is a great article that explains how to change the recording levels:
Note: adjusting the volume of an audio file in GoAnimate does not affect lip movements. The adjustment must be made prior to importing.
Learn more about Audacity and other third-part tools.