Pencil2d doesn’t allow you to delete such a sound layer from inside a project.
The only solution is to push the unwanted sound file beyond the end of the project!
With very small projects Pencil2d allows the deletion of the sound layer.
I’ve now found a way to delete the sound layer.
I saved the .pclx file from my Mac to a memory stick
Plugged memory stick into PC Windows 10
Changed file extension to .zip
Extracted all, then edited the main.xml file to remove layer
Wrote edited main.xml over unedited main.xml in zip file
Renamed .zip file as .pclx and transfer back to the Mac
When loaded the required sound layer had been removed.
PEW!!!