Thanks for your reply Lord.
Thats what I would have thought. When filtering All files, both audio and video are listed and against each file name, it is shown in the type column as either audio or video.
When filtering only to audio files, there is no listing and when filtering only to video files, all these files are shown.
Following your comment as to whether there is some confusion in the type as to mp3 or mp4 files, I check audio files to see whether they were coded correctly. However, they are all coded correctly as mp3 files.

Originally, when I started downloading, these files were shown correctly, but somewhere along the line, the audio filter now does not show any files.

Curious indeed! Any other thoughts?

Tiptop