Thank you for the info!
The MP3 file in this case was a transcode of the WAV file (honestly, when I was doing this I didn't have a list of compatible audio containers, and I figured MP3 would be more compatible) - in this case it was also transcoded from 44.1KHz to 48KHz as I also wasn't sure how well Lumafusion would handle multiple sampling rates on the audio (the video file contained 48KHz audio out of FilmicPro). I did late find that Lumafusion seemed more than happy to work with the 44.1KHz WAV file
Unfortunately, I can't use your technique for syncing - although it is almost identical. As the audio is recorded directly on the digital piano (USB drive plugged into the piano where it creates the master audio file, there is no way to use a clapper in this situation as the recording on the piano would never record that sound - what I normally do is record a group of quick-hit keys so I have brief peaks (usually a set of 3 to help with alignment and be able to hear for echo better) then I either record everything then strip out the camera audio for the piano portions once synced OR the piano audio may not even be picked up by the mics on the camera (like if a headset is plugged in).
I only have to usually sync that one external track, so I rarely have a complicated syncing situation (granted, I've been spoiled by FCPX with its auto-align on sound marker which works well - but I had been doing it with the quick staccato note hits (although sometimes it's not as sharp as I can make it
so this isn't a terribly new technique for me
Technically, it's the same type of method conceptually as the clapper, but the peak isn't quite as sharp as a clapper would create (read not as sharp as all, but there is a good demarkation that makes alignment fairly easy, it almost feels like I'm being restricted to frame alignment).
I'll try trimming the audio on the audio clip to see how that works.
It would be nice if one could shift a specific clip without it scrolling to the beginning of that clip every time