Easy Noise appEasy Noise is Multimedia tool that is open source under the GPL-3.0-or-later license. Continues playing even under other audio. Includes a play/pause widget, a Quick Settings tile, and seven sound options.
This app does not contain any ads or trackers. It does not ever connect to the internet. It does not store any data of any kind other than the current volume level and selected sound.
The app needs the "Phone" permission ("Allow Easy Noise to make and manage phone calls?") ONLY to be able to pause playback during phone calls. The usual method for handling this involves something called Audio Focus, but that would mean that this app would not be able to play noise beneath other sounds (such as music or video).
The full source code is available at https://github.com/cliambrown/EasyNoise