Hi Martin,

Are you using TS files in Premiere by any chance?
If so, then Premiere has trouble syncing some TS file formats, you can confirm this by playing the files
in VLC instead and they should be in perfect sync.

The solution will be to look at a different video format. MP4 with hardware encoding would be my pick.

Regards,

Martin
vMix

Not an issue. SDI cable length does not affect audio sync. When the overall cable length gets too long you will see general signal break-up/dropout, or total signal loss, but not a deterioration of audio sync (while the picture remains perfect).

Professional 3G/HD/SDI cables with Neutrik connectors are garanteed up to 230m (= 750ft or 250yards) for FullHD signals with no audio or video loss or desync.

Concerning your out-of-sync problem, I was wondering where does your audio come from? Your cameras or an audio mixer?
Can you indicate a value for this delay? Are we talking milliseconds, hundreds or tenths or even seconds? Many acoutic factors may delay audio which by default transports much slower than light...

Yes it's a good method to do clapping tests in close ups or with lips "popping" (saying P with air)

I'm quite sure both options for MP4 are pretty much the same, except FFMPEG allows more control/choices. BTW whatever the encoders, the containers or the codecs, I never noticed any out-of-sync with vMix in years.