Extracting Audio from .mp4 creates weird/invalid .wav file
Some background: This clip was recorded by a Sony AX53. Sony abuses the .mp4 container format, and the audio encoding format the cameras use is not technically supported by the .mp4 container format. They call it XAVC. Still, it works, with some side effects.
When cutting sections of the clips, the audio needs to be converted into a format that the container supports, as MLT does not want to write non-compliant media files.
Up until very recently, extracting audio from these clips worked fine, but now when extracting the audio, Kdenlive creates the .wav file, but then does not create the audio thumbnail for it, and does not allow it to be dragged into the timeline.
Audacity happily edits this .wav file, and when it is exported as an .mp3 file, Kdenlive happily accepts it and reads it.
So, the wav file produced when extracting audio from these video files are valid .wav files, and as mentioned earlier they worked just fine until very recently in Kdenlive. As Kdenlive can read these clips with no issues, and is already converting the audio to .wav format when extracting it, there is no reason for it to make a file that it has trouble reading.