@Unmutual
Would you be able to test with the files from Video Audio Sync Tests @ 23.98, 24, 25, 29.97, 50 & 59.94 and confirm that you are seeing perfectly in sync audio with those files (in particular the 23.98 fps one? The files make audio sync issues more clear to see compared to some movies.
When testing, also try seeking in the file after it has played a while and confirm if you see any change in audio-sync
Also how are your remuxes played back? Local hdd, smb over a local network, … etc
Thanks for the input on a problem that you are not seeing