I'll try not to make this long-winded - but I thought I'd share the process I went through and the two solutions.
Camera footage from two Canon C100s (23.98 fps AVCHD) rewrapped to MOV using EditReady, metadata maintained.
Audio from a Sound Devices recorder (perhaps a 633 or 644) Camera guide track audio - 48kHz
Audio Specs: Polyphonic WAV, sampling rate 48kHz*
Problem: Sync drift when syncing footage in FCP X; in sync at beginning of clips, progressively and increasingly out of sync towards end of clips.
Confirmed that storyline settings were matching.
Confirm FCP X was reading all format/metadata correctly.
Verified audio format in WaveAgent app. Tried changing timeline to solid 24FPS - still out of sync.
Tried syncing in DaVinci 14.1.1 - still no luck.
Used PluralEye 4s for sync-drift correction feature Using FCP X sorting and show in Finder feature, I located each grouping of syncable video and audio clips and put those into PluralEyes 4 - one grouping at a time. PluralEyes corrected the sync drift about 85% of the time.
Sent the multicam clips from PluralEyes 4 to FCP X
Roadblock:Finished all of the syncing and then realized a major problem: PluralEyes mixed all of the channels of my polyphonic file and threw out the metadata. For those of you who have read some of my posting, you know I am not a fan of PluralEyes for many tested reasons, but I used it for this one sync-drift correcting feature.
Next Steps: I need those channels back. Retime tool to the rescue
I had this idea of lining up the original audio with the PluralEyes corrected audio. Added an angle to multicam clips in FCP X or attached new audio to synchronized clips in FCP X.
Lined the original audio (which has the channels and metadata) against the corrected audio from the head edge.
You have to zoom way in on the time line to do this: Went to the end of the clip and used the retime tool on the original audio (which was typically longer than the sync-drift corrected audio) and dragged the retime tool so that the original audio tail matched the sync-drift corrected audio tail edge. Sometimes I still had to drag a little more looking at the waveform of both to match. The changes were so slight that the retime tool didn't even register a percent change (from 100% to 99%)
Repeat for each syncing group - yeah a lot of work.
A Friend to the Rescue.
Contacted a friend who's more adept with all things sound - shout out to Jim Schaefer. We discussed the issues and the process.
Jim's thoughts: A)Recordist thought we were shooting 24 fps and finishing in 23.98 so he did a trick to prevent us having to do a pull-down on the audio to match the telecine'd footage. He forgot to reset the sample rate.
Jim's revelation: There's another setting Sound Devices has for changing the sampling to 48.048kHz. Try resetting the sample rate of the original audio to 48.048kHz in Wave Agent and see if that works.
Tried Jim's advice and synced the changed clip in FCP X. VOILA! IT WORKED.
Conclusion and Revelations
All I would have had to do is change all of the audio to 48.048kHz and all of the audio (whether by FCP X automatically or me lining things up by slate) would have synced. And they did as I went back through this process; changing [a copy of] all of the original audio to 48.048kHz in Wave Agent and adding them to their appropriate multicam or synchronized clips in FCP X
Why the recordist used this sampling rate, we don't know.
Why this information wasn't communicated to us, we don't know.
FCPX doesn't care about frame rate metadata on audio clips and try to adjust audio files based on FPS information.
Time to get some Tentacle Sync devices or something similar