Multi-Cam audio sync issues

I've made many multi-cam videos with my camera rig and was stumped last night when I came in from a dress rehearsal of a chorus concert to find that (unlike dozens of shoots in the past) audio would not sync on two clips although I could see visually where the tracks match. I ended up using in-point sync (cameras do not make time code). Back, in other words, to 2015 or so before sync on audio was available, and I need to use visual cues in the audio track to sync. 

I'm HOPING my problem is that although there is plenty of audio data in my clips that match, because I was recording a dress rehearsal with little prep time I started the cameras with a piece of music (chorus and orchestra performing Haydn's PAUKENMESSE) in progress rather than ahead of the first music. 

I'm normally recording a play or opera with a period of silence, applause (conductor walks in) maybe or the curtain goes up and actors start dialogue. In this instance, I started with music in progress. 

I can't find documentation on how to do a successful audio sync - it always worked before. Not having it work is unusual and a bit scary. If anyone has insight I would really like to know! Thanks all...

CAMERAS: Sony FDR-AX100/HXR-NX80 (identical outputs and very similar electronics; I can control the NX80 remotely with an iPad where I need to adjust focus, iris and/or zoom during the shoot).

COMPUTER: PC, i9-9600, 64G RAM, RTX-2070 video.  

Software is current and up-to-date.