Festival saves all of its files within the "OS data" directory.

You can delete this folder to reset Festival's state/settings and the Collection, or more easily, just run:

./festival --delete
macOS$HOME/Library/Application Support/Festival/gui/Users/Hinto/Library/Application Support/Festival/gui
Linux$XDG_DATA_HOME/festival/gui or $HOME/.local/share/festival/gui/home/hinto/.local/share/festival/gui

Festival saves everything into gui because other frontends will use the same festival project directory, e.g:

├─ gui/
├─ daemon/
├─ cli/

The sub-directories and files within gui/ and their purpose:

├─ gui/
   ├─ state/
   ├  ├─ audio.bin      # Audio state, e.g: elapsed time, current song.
   ├  ├─ collection.bin # The main music `Collection`, holds metadata and PATHs to audio files.
   ├  ├─ playlists.bin  # The `Playlists` database, holds all info about all playlists.
   ├  ├─ settings.bin   # `GUI`-specific settings, e.g: sorting methods, album size.
   ├  ├─ state.bin      # `GUI`-specific state, e.g: current tab, search input.
   ├─ docs/  # Documentation files from `festival --docs`
   ├─ image/ # These are resized images of the album art.
   ├         # Festival itself does not need these (all images are
   ├         # within `collection.bin` itself) however, Festival hooks
   ├         # into the OS's native controls which needs a PATH to an
   ├         # image to display it. These are for that.
   ├─ signal/ # This is how Festival communicates with an existing one
   ├          # (e.g `festival --play`), via filesystem-based signals.
   ├          # `festival --play` quite literally just creates an empty
   ├          # file inside this folder called `play`.
   ├─ txt/
      ├─ crash.txt  # Crash/panic data. Useful for bug reports.
      ├─ perf.json  # Collection creation performance timings, in JSON.