Tutorials
How to Merge Audio and Video Online
Replace or mix audio tracks with your video file — all on your device.
Replacing bad camera audio, adding a music bed, or syncing a voice-over often means combining separate files. With Merge Audio & Video, you upload a video and an audio track, choose replace or mix mode, and export one MP4 — in the browser with no cloud upload step.
When to merge audio and video
This workflow fits when you:
- Recorded video on a phone but captured clean audio on a mic or recorder
- Need to swap a noisy track for music or narration
- Want both ambient room tone and a new track at reduced levels (mix mode)
Use Video to MP3 if you only need audio extraction, not a new combined file. Use Merge Videos when you are joining multiple video clips, not attaching external audio.
Step-by-step: merge tracks with ALTools
- Open Merge Audio & Video and upload your video file and audio file (MP3, WAV, M4A, and other common formats).
- Choose Replace original video audio track to drop the built-in sound entirely, or Mix with original audio to layer both (available for smaller files).
- Adjust Audio offset if the external track starts early or late. Scrub the preview while nudging seconds until lip sync looks right.
- Enable Use shortest track duration if you want output length to match the shorter input — useful when music is longer than the clip.
- Optionally turn on Optimize for maximum compatibility (takes longer) if players reject the first export.
- Click Merge & Download and wait for local encoding to finish.
Both inputs stay in the browser tab throughout the merge.
Replace vs mix: which mode?
| Mode | Result | Best for |
|---|---|---|
| Replace | Only the uploaded audio plays | Voice-over, music beds, fixing bad camera mic |
| Mix | Original + new audio combined | Keeping room tone under narration |
Mixing may be unavailable for very large files because of memory limits — the UI explains when that applies.
Level matching before you export
Replace mode ignores the original track entirely — if your uploaded narration peaks much louder than the old camera mic, viewers will notice a sudden jump. Listen on headphones and normalize the external audio in a DAW first when levels swing more than a few dB.
In mix mode, start with the new track slightly lower than you think you need. Room tone from the camera can mask thin narration; pushing music too high buries consonants. Export a ten-second test, then adjust offset and levels before committing to a forty-minute file.
If the video has no audio and you use replace, the output length still follows the video unless shortest track is enabled — double-check which input should win when music runs longer than picture.
Sync and timing tips
- Clap or use a visual cue at the start of both recordings to align offset faster.
- Negative offset pulls audio earlier; positive offset delays it.
- Export a short test section with Trim Video if you need to verify sync before processing a long event recording.
- Clap sync — one visual clap on camera plus a spike in the external WAV makes offset obvious when scrubbing waveforms mentally.
Wedding videographers often replace on-camera audio with lavaliers — replace mode is default; mix mode is rare unless ambient room matters.
Common issues
No audio in output. Confirm you selected the correct audio file and that replace/mix settings match your intent.
Mix option disabled. File size exceeds the mix-capable limit — try replace mode or compress sources first.
Video has no audio track and replace is on. That is expected — output uses only your uploaded audio.
Merge failed on large files. Close other tabs, use a shorter clip, or enable compatibility transcode.
Frequently asked questions
Can I keep part of the original audio?
Yes — use Mix with original audio when available instead of replace.
What format is the output?
MP4 for broad playback compatibility.
Related tools
- Video to MP3 — extract audio only
- Trim Video — shorten before merging
- Compress Video — reduce final file size