(UPDATED: It now contains ALL the sounds that are not in any of the regular categories, and NO sounds that are in any of the regular categories) Click the buttons to play the sounds. The name of the sound being played is displayed at the bottom of the screen. The semi-transparent buttons indicate that the sound has tags (which can be used to search for sounds). The tags are in brackets after the sound name at the bottom of the screen.
This is a library of the "hidden" sounds in the Scratch library. They are not missing from the Scratch library itself, but they are not included in any category, so you can only find them in the "All" category, which is long and can be tedious to browse through. Most of them have no tags at all. Those that do are semi-transparent, and have their tags written in brackets after the sound name. Some of the sounds are included in sprites from the sprite library, such as the owl sound. I have tried to sort them into the categories that I think are the most relevant, but several of them actually fit into multiple categories. For example, beatbox sounds are both percussion and voice. Thanks do @donotforgetmycode for directing me to the source code JSON file containing all of the sounds and their tags, which allowed me to make this project perfect. https://github.com/LLK/scratch-gui/blob/develop/src/lib/libraries/sounds.json