When you combine separate audio and video files, they rarely have exactly the same duration. Understanding how tools handle this mismatch prevents surprises in your output.
Scenario 1: Audio is longer than video
Your video is 120 seconds. Your music track is 180 seconds. The output file must have a definite length. Three possible behaviors:
Truncate audio to match video (most common): The output is 120 seconds. The audio plays for the first 120 seconds and the remaining 60 seconds of music are discarded. The video plays in full. This is the most practical behavior because you usually want the full video with music underneath.
Extend video with freeze frame: The output is 180 seconds. The video plays for 120 seconds, then the last frame is held as a still image for the remaining 60 seconds while the music continues. This is rarely desirable unless you intentionally want a frozen ending.
Truncate output to shorter: Same as truncating audio. The shorter stream determines the output length.
MediaBrew truncates to the shorter of the two. If your music is longer than your video, only the first portion of the music plays. Choose a music track that is close to your video length for the best result.
Scenario 2: Audio is shorter than video
Your video is 120 seconds. Your voiceover is 90 seconds. The last 30 seconds of video would have no audio. Possible behaviors:
Pad audio with silence: The output is 120 seconds. The voiceover plays for 90 seconds, then 30 seconds of silence accompany the remaining video. This works well for narrated videos where the final segment is a visual conclusion.
Truncate video to match audio: The output is 90 seconds. The video is cut at the 90-second mark to match the voiceover. This may cut off important visual content.
Loop audio: The audio repeats from the beginning to fill the remaining duration. This works for background music but sounds jarring for voiceover (the narration would restart mid-video).
Scenario 3: Audio and video are nearly the same length
Your video is 120 seconds. Your music is 118 seconds. The 2-second difference is barely noticeable. Most tools handle this gracefully by truncating to the shorter duration. The last 2 seconds of video have no audio, which is imperceptible in practice.
If the difference is larger (10+ seconds), it becomes noticeable. Plan your audio and video durations to match within a few seconds.
How to match durations before combining
If your audio is too long: Trim it using an audio editor or let the tool truncate automatically.
If your audio is too short: Either trim the video to match, loop the audio (works for music, not voiceover), or accept the silence at the end.
If you need precise synchronization: Use the watch-and-narrate technique (narrate while watching the video in real time) so the voiceover duration naturally matches the video.
Checking duration before combining
On Windows, right-click a file and check Properties > Details > Length. On Mac, select the file in Finder and press Cmd+I to see duration. On any platform, uploading to a MediaBrew tool shows the file duration in the interface.
Knowing both durations before combining prevents surprises in the output. The MediaBrew add audio tool truncates to the shorter duration automatically. If you need to adjust lengths first, use the trim tool for video or trim your audio in any audio editor before combining.