We shot a short film a few days ago using the Arri Alexa. We used 25fps at 2k resolution, with no onboard audio recorded into the camera. We recorded audio separately. When syncing up the audio with the clapperboard the start of the clip is in sync, but as the clip goes on the audio drifts away leaving it out of sync. The sequence settings are matched with the 25fps, what on earth is going on here?