audio and video unsync when exporting

I've recently noticed that when exporting projects my audio shifts one to two frames before the video itself and leaves a mute space at the end due to the audio coming before. The videos that I use come directly from youtube and are not detached from their source, I'm essentially just cutting clips from various different videos and exporting them. I haven't changed any of the project and export settings at all, and use the default 48khz that it creates a project in. The only difference that I have made that I can think of is upgrading my laptop to Big Sur and redownloading final cut pro. Would greatly appreciate it if someone had any idea of what is going on and how to fix this issue.

What is the sample rate of the original audio? It must be 48kHz. If not, change project settings to match.

lia wrote: ...mute space at the end due to the audio coming before. The videos that I use come directly from youtube and are not detached from their source...I...use the default 48khz that it creates a project in. The only difference that I have made that I can think of is upgrading my laptop to Big Sur and redownloading final cut pro...

This might be caused by variable-frame-rate material such as smartphone, Zoom, screen capture, Excel video export, etc. Or it could be something else.

Temporarily discontinue any use of wireless audio such as Bluetooth earphones, or any wired outboard speakers. Use only the built-in Mac speakers while investigating. Also do not use any outboard video processing such as extra monitors. Those can have unpredictable and variable video processing delays which only appear under certain circumstances.

Next reboot the machine and make sure you have plenty of extra disk space. We've had several cases where a person would work on a problem for days only to find they'd never rebooted or checked their disk space.

Then pick out a place where the mis-sync consistently shows up. On Youtube inspect that closely via various playback methods: Youtube using single-step. In the web browser Youtube player, do SHIFT+? to see the various playback commands.

Then download the Youtube file and single-step that same region using the JKL keys in Quicktime Player.

These are all to absolutely verify the delay does not exist before importing to FCP.

Once in FCP, just use the one clip with the delay in a separate project. Apply no Fx or any other edits. See if the delay happens during FCP playback. If it does, transcode that clip to optimized media and try again.

If it still doesn't happen, export the clip to ProRes 422 via File>Share>Master File>Settings, Format: Audio & Video, Video codec: ProRes 422. Play the exported file in Quicktime Player and evaluate.

If it still doesn't happen, export using these settings: File>Share>Master File>Settigs, Format: Computer, Video Codec: H.264 Faster Encode, Resolution: (whatever the original resolution was). Play the exported file in Quicktime Player and evaluate.

If it only happens when exporting from FCP, open the original file in Quicktime Player, trim the file and export to a different resolution. This will force it to re-encode the output. That will help differentiate between an edit done by FCP vs one done by Quicktime Player. For instructions on trimming with Quicktime Player, see this video: