@B3Guy My workaround was to transcode this particular file to ProRes in EditReady and import that into the project - clip plays back normally now.

It has been figured out. I'm relieved to say, it wasn't the NX1's fault at all, nor the fault of iFFMpeg's converting. The editor was importing the files into FCPX with the option to "remove silent channels" checked, and FCPX was removing good channels on longer video clips. (I'm shooting weddings so obviously that would be the important clips like ceremony/toasts/dances.)

The only other issue I've encountered in several years with the NX1 as my main camera is occasional corrupted files. It seems to happen if the REC button initiates a clip and then is pressed very soon after (<4sec). These ultra-short clips become corrupted, and that wouldn't be terrible except for the camera will often freeze up when trying to initiate recording of the subsequent file. I suppose that the corrupted file is confusing the sequential file numbering system somehow but IDK. Pulling the battery to reboot doesn't always clear the issue, as the file recorded that freezes the camera will also be a second corrupted file, perpetuating the issue. Taking a picture instead of a video often helps, or just being very patient with the camera when starting up . . . switching into movie mode . . . and . . . hitting the REC button.