I’ve recently discovered a problem with sound layers. I got myself out of trouble because I do incremental saves. That is I save my projects regularly using filename, filename-2 etc. This allowed me to recover the situation.
If you put more than 1 sound file per sound layer, you can’t delete the second or subsequent sound files from they layer, or delete the sound layer without Pencil2D crashing.
Users need to be warned about this problem.
I’ve just discovered another wrinkle, if your project has no bitmap or vector layers and just consists of a camera layer and a sound layer, you can delete a sound layer with more than one sound file.