AI Dubbing: When Voice-Only Is Enough and When You Need Lip Sync

AI dubbing translates the speech in a video and replaces it with a new voice track in another language. Most tools now offer two versions of the job:

• Voice only swaps the audio and leaves the picture as it is.
• Voice + lip sync also changes the mouth so it matches the new words.

The second sounds strictly better, but it isn't always the right choice. It costs more, takes longer and has limits of its own. Here's how to choose.

What changes between the two

Voice only is an audio job. The pipeline transcribes the original speech, translates it, generates new speech (often trying to keep the original speaker's voice) and mixes it back over the background sound. The video frames aren't touched.

Voice + lip sync adds a video job on top. The model regenerates the mouth region frame by frame to match the new audio. That's where the extra time and cost come from.

As one concrete example, FunGen's dubbing tool charges 3 credits per second for voice only (up to 5 minutes) and 11 credits per second for voice + lip sync (up to 120 seconds). In their published tests on 7–8 second clips, voice only finished in about 17–29 seconds and lip sync took about 2–7 minutes. The exact numbers differ between tools, but that ratio is typical: lip sync is a several-times-bigger job.

When voice only is enough

• The speaker's mouth isn't the focus. Screen recordings, product demos with a small face-cam, B-roll with narration and slideshow-style tutorials all fall in this group. Nobody is watching the lips.
• The video is long. A 20-minute course module is a lot of lip-sync seconds. Dub the voice, and save lip sync for the intro where the instructor is on camera.
• You're testing a market. If you don't yet know whether Spanish or Portuguese viewers will watch, voice only lets you try several languages cheaply before you commit.
• There are several speakers at once. Lip sync works best on one clear face. Panels and interviews with crosstalk often come out better with voice only.

When lip sync is worth paying for

• Talking-head content. Explainers, founder videos, UGC-style ads and avatar videos are all close-ups of a face. A mismatch between lips and words is obvious within a second, and viewers read it as low quality.
• Short-form social video. TikTok and Reels clips are short, so the per-second premium adds up to little, and the face usually fills the frame.
• Ads. If the video is meant to convert, the uncanny look of unsynced lips is a real cost.

Things that go wrong either way

Length drift. Languages don't take the same time to say the same thing. Spanish and German often run longer than English. Lip-synced output may come out slightly longer than the original. In FunGen's own tests, 7–8 second English clips came out about 1.3 seconds longer in Spanish. If your video is cut tightly to music, leave room for this or re-edit afterwards.

Messy source audio. Background music, crosstalk and strong accents hurt the transcription step, and every later step inherits the mistake. A clean, single-speaker source is the biggest quality lever you have.

On-screen text. Dubbing doesn't translate captions, titles or text burned into the picture. Plan to redo those separately.

A practical workflow

1. Cut the source to the version you actually want to publish before dubbing. You pay per second.
2. Dub one short section in each target language with voice only, and check the translation with a native speaker if you can.
3. For the languages that pass, decide clip by clip: lip sync for on-camera segments, voice only for everything else.
4. Re-check timing against the music and on-screen text.

If you want to try both modes side by side, the AI dubbing tool in FunGen covers 32 languages and shows the full price before it starts.

One rule matters more than any setting: only dub videos you have the rights to, and only recreate voices of people who agreed to it.