I've noticed several TV commercials that start with a voice over, and later cut to the narrator. There is a unique audio effect, that makes the voice still sound like an overdub, but the lips are in sync. How is this done?
Most ads use ADR for the audio.
@gauss256 is correct; the easiest way is to record both audio and video of the narrator at the same time. However, sometimes this is not the best option for the audio. For example, if the narrator was walking along a beach while speaking, the recoded audio would not be clean. So in that case, they would record the audio as a guide track and then get the actor into a studio for an ADR session. This way they can record perfectly clear audio which they can edit to obtain that 'authoritative' 'voice of God' type sound.
As for the example you posted in your question, I'm not too sure if that's ADR or not. There's only about a second of footage of the narrator so it's difficult to see if his lips match perfectly with the audio. However, as the background is just pure black, I would assume they'd have recorded his sound and vision in a studio – so as to kill two birds with one stone.
In terms of how to achieve that 'voice of God' sound, I couldn't give exact instructions. You would just have to experiment with pitch shifting and equalizers in your chosen audio editing program.
That typical kind of sound is usually achieved by recording the voice in the near-field of a large-diaphragm condenser microphone. That gives you
- very little room influence, so it sounds very direct.
- strong, but focused bass response due to the proximity effect.
Since this recording technique requires the speaker to have a fat microphone and pop killer right in front of their face, usually no video is taken from such recordings; as it's so short it's really not a problem to do that with ADR, which they probably did in this clip.
The easiest way is to just record both audio and video of the narrator. During editing, use the entire audio track from that recording but just a portion of the video track. It's known as an L cut.