I think I figured out what the problem that triggered that error message was. My video camera records audio at 48khz. My DAW (Reaper) and audio interface were set to 44.1khz. In Reaper the sample rates can be mixed and matched, no big deal, it figures it out, so I didn't think to make sure Reaper and my audio interface were set to record at 48khz before doing these test run videos and audio recordings. Apparently Kdenlive and Shotcut can figure it out with different sample rates, too--neither had any trouble doing a sync, regardless of how many clips I was working on. (Audio for this project is three main tracks with 15 clips per track, plus a couple overlay tracks with just one or two.)
But in Videopad, if I imported the audio at 44.1khz and the video at 48khz, I got that error, "No matching sound track found." But after converting the audio files to 48khz with Reaper, then importing the same clip, the sync on a single mic clip compared to a single camera clip worked.
So it seems that Videopad will only sync audio if the sample rate matches. I suspect the reason Videopad was able to do some sort of sync attempt when all the files were in one sequence, but took forever and botched it badly, is because it did find one other 48khz clip, probably from my second camera rather than any of the audio-only tracks.
I also did a test both on this computer and on a second computer with a fresh Videopad installation where I put 3 of the camera clips and the matching, resampled 48khz audio-only mic clips into a sequence. Selecting either just one or all of the mic clips and trying to do Audio Sync took a while but it appeared to work--however, on listening, only the first clip I selected actually ended up in sync. The second was audibly out of sync like a very short delay effect where it overlaps but sounds weird, and the third was way out of sync like a long delay effect with totally distinct repeat. If I selected the second clip and tried again, that clip got synched, but then the first that was previously synched got out of sync.
And my final test was using one clip from the main camera, one clip from the mic, and one clip from a second camera (phone). In that case the mic synched with the main camera, but the phone track remained way out of sync.
So as suggested by earlier results and our prior discussion, it does indeed appear true that only one clip can be synched to one matching clip at a time--any attempt to go beyond that, either in terms of other sources or other takes, fails to give usable results. Which means to work with 15 clips I'd have to make one or more sequences just for synching, then move clips over to the main sequence one at a time, and when working with a third track, manually place that track's clip in the main sequence over the other two related clips according to the sync results.
The apparent "sample rate has to match" limitation of the audio sync feature doesn't appear to be mentioned anywhere that I can find--but it's pretty important, so if it's an intended limitation, better add it to the help files. If it's unintended, I'd call that a bug.
Same goes for the problems with synching multiple files at once--that's a pretty big limitation vs. the open-source editors I tried, and it needs to be clearly stated up front to avoid wasting time on this sort of troubleshooting, or to be fixed if it wasn't the intended behavior.
Either way, it's an area that needs improvement in Videopad, as it looks like it will be much faster to work in Kdenlive or Shotcut for the initial stage of simply synching up sources. Not sure yet about other types of edits like overlays or pan/zoom/crop effects--I actually think the most user-friendly version of the latter group is in Photostage, unless you need to enlarge.
