I’ve been testing AI avatar videos for YouTube Shorts and one thing I noticed is that most bad-looking HeyGen videos are not caused by the avatar itself, but by poor voice timing. If your AI voiceover has long pauses, uneven pacing, or too much background noise, the lips start drifting after a few seconds. The cleanest results I got in 2026 were using 44.1kHz WAV audio with slight compression before upload. Also avoid robotic TTS voices with ultra-fast speech because the phoneme detection struggles during quick consonants. HeyGen’s latest Avatar IV system improved expression syncing a lot compared to older versions.

One trick most people miss is splitting long scripts into 20-30 second segments. The sync quality stays much tighter. I used to upload 4 minute narration files and the mouth movement slowly became unnatural near the end. Smaller scenes render cleaner and faster.

I tested voices from ElevenLabs and they sync way better than many built-in TTS voices because the speech cadence sounds human. Slight breathing and pauses actually help the avatar realism now.

For anyone editing audio first, normalize volume before upload. I use this FFmpeg command:

ffmpeg -i voice.mp3 -af loudnorm output.wav

After doing this my lip sync accuracy improved noticeably, especially on multilingual videos.

Another hidden setting is eye contact and gesture intensity. If gestures are too aggressive while speech is calm, viewers subconsciously feel the sync is “off” even when lips match perfectly. Keep movement style close to voice energy.