I recorded a video on my camera with separate audio on my phone's voice memo app, and by the end of a 20-minute clip they're noticeably out of sync. Is there a way to fix this in post without manually stretching the audio track? I'm using DaVinci Resolve.
The fastest fix in DaVinci Resolve is to use the Retime tools to stretch your audio clip to match the duration of your video. Import both into a timeline, then right-click the audio track and select "Retime" - you'll get speed controls that let you adjust the audio duration to match your video without manually syncing frame-by-frame. This works well when the drift is gradual across the whole clip, which is usually what happens with separate recordings.
If you want more precision, you can use audio sync markers. Clap your hands or make a loud sound on both recordings at the start (or identify a moment that's clearly visible and audible in both). Line them up visually at that point, then check the end - if they're still drifting, the retime method above is your answer. Just calculate roughly how much slower or faster your phone's audio ran. If the audio ends about 2 seconds late on a 20-minute clip, you're looking at maybe 0.5% slower playback, which you can dial in pretty precisely with Resolve's retime controls.
As a last resort, some people use third-party sync software like PluralEyes or Sync! (which works as a plugin in various editors), but honestly for a single drift across one clip, stretching in Resolve saves you the trouble and cost. Just make sure you're working on a copy of your audio so you don't mess up the original file.
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