Audio visually disappeared but still heard?

The "audio" you see in the track isn't really a picture of your actual audio, but rather a picture of a decimated copy of the audio, stored in the Picture Cache folder.

That one folder serves all projects, so if you look in it you'll see hundreds or thousands of files, each named for the project they belong to. If you start a new project by loading a template that has saved audio in it, that cached image will not be associated with the new project. No picture file, no image in track view.

The fix is simple: right-click on the audio clip and select Clip Properties. Go to the Audio Files tab and click on the "Recompute Picture(s)" button. If you have multiple clips, you can select them all and refresh their pictures all at one go.