The Game Creators has released a Sound Library for AppGameKit, the first of its kind for the product. It features a total of 250 unique sound effects that are royalty-free.
It is unclear what the actual license for this sound library is and without a way to preview them, this purchase can be considered a gamble. It can be found here.
Source: https://www.thegamecreators.com/post/appgamekit-new-sounds-library-dlc-now-available