I [again] ran into this issue today and might have finally found a solution. It is a little hacky to get there, but that's okay!
TLDR at bottomA little preamble: we shoot dual system, so I have video files with multiple separate tracks of audio (boom, lavs, mix, etc.). Instead of jamming timecode, our setup relies on the on-camera mic or a feed from the sound recorder to give an okay enough scratch track to sync to. But sometimes the scratch track just isn't possible, eg. the recorder is too far away, scratch track is noisy, etc.
I much more prefer to have clips synced in my bins as "standard" video files rather than multi-cam clips (unless we're truly running a multi-camera setup) or other solutions like leaving them as linked tracks in a timeline and constantly grabbing from there.
To achieve this, most of the time "Auto Sync Audio" from the Media Browser in the Edit Page gets me what I want. BUT when the auto sync fails, I'm usually stuck using a multi-cam solution. Every so often I check the forums if there has been any changes to the software or solutions that I hadn't seen, but I hadn't yet found anything. However, just recently I tried a different (and in hindsight kinda obvious) approach. I consider it hacky because I think there should be a way to do this in one step, but if it works then I'm okay with it!
Instructions:
- In the Media Page, sync ONE audio file. There are many tutorials on how to do this exist, but basically you select your video clip in the browser and move the playhead to the first frame of the slate's closure; then select the "best" audio clip and line up it's playhead (on the right hand side) to its waveform spike at the slate's clap; then finally click on the little 'link' icon in the lower right of the audio area.
** Your video clip should now had new audio. This new "merged" clip can be used to sync the additional audio channels (ie. audio clips). - Now, as if you had a good scratch track, select the new merged video clip and then command-clicked the other audio clips. Right-click the video clip (with the others still selected) and choose Auto Sync Audio. When prompted, I selected Waveform as the sync method. Voila!
- Go into the Clip Attributes and change the audio layout to make it work for you. For me, my new layout was 1) Mono: BOOM (linked) 2) Mono: LAV (linked) 3) Stereo MixL (linked) / MixR (linked).
** I usually do steps 2 and 3 in the edit page so I can drag the finished synced-clip into a timeline to test my work)
My solution TLDR: to sync multiple audio clips to a single video clip that can't Auto Sync, first sync
one audio clip to the video clip in the Media page using the manual sync method; then use Auto Sync to merge this newly merged clip to the remaining audio clips. Adjust your Clip Attributes as appropriate and then test on the Edit Page.