I have converted my video files from .MTS to ProRes (Proxy). When I play them back in QuickTime everything is fine. When I import them into Final Cut pro, they start getting out of sync with the audio at the end of the clip (they're about 20 minuts long).
I'm at a loss why this is happening. There is no problem in QuickTime. Why would there be in Final Cut Pro. Final Cut tells me that all files are 48,0KHz, 16-bit integer.
What do the original files show up as in a media app such as VLC? Does this info match what Final Cut says about the ProRes copy? The video frame rate, audio sample rate, and other information should match exactly. Then, your FCP sequence settings should be identical.
Another way to address small sync drifts is to razor-cut the audio and manually re-establish sync a few times on the project.
I had the same problem. It seems to be a ProRes issue. I fixed it by converting the audio to AIFF (ffmpeg -i video.mp4 -vn audio.aiff) and importing the new audio file into FCPX separately, then creating a compound clip with the video and audio.
I had a similar problem with Fina Cut and Soundtrack on a 24fps timeline. There seemed to be some mixup with the audio in 23.97fps and the video in 24fps. I had to apply a time stretch to get it synced.
