My company does a lot of ProRes video files where we take a video file with Stereo and ME audio tracks and sync it to 5.1 tracks provided by the client and then export a new file. These video files are either captured from tape or provided as files already. The provided frame rate varies but is usually either 25 or 23.98. The issue I keep getting into is that I very often can't get the 5.1 to sync to the original video. We always start with the original video when doing sync whatever it was provided as so if it's 25 we try to sync it to the audio tracks without changing frame rate. Nothing I seem to do other than going through the audio and editing it to match the video seems to work consistently.
I am aware that sometimes Final Cut assigns a frame rate to an audio file even though it has no frame rate and have done the work around imported and exported it out in QT Pro and then reimported.
I also am aware that there's not really a full story in this post as we never know or are able to verify what files these 5.1 files were created to sync to. I just feel that if it happens once or twice, there is an issue with the files we're given. Every time or almost every time, the issue may lie with us.