Sleep like a Pancake! is a simple, open source, ad-free, mildly-mindful sleep sound app utilizing audio files from Freesound.org [1], mindful meditations from The Free Mindfulness Project [2] (mostly) and noises generated by the author via Audacity [3].
It was created with Flutter [4].
There aren't many. A sleep app should get out of your way and not frustrate you.
Slap is open source software and it's audio files are release under Creative Commons Licenses.
Links:
[1] https://freesound.org
[2] http://www.freemindfulness.org
[3] https://www.audacityteam.org/
[4] https://flutter.dev
[5] https://play.google.com/store/apps/details?id=org.pancake.slap
[6] https://github.com/ryanlath/slap