SoundAura is an open source ambient sound app. It does not include any audio tracks to save on download size, but allows users to add local files from their device and play any (reasonable) number of them concurrently, with individual volume controls for each track. SoundAura can also switch between an audio focus respecting mode, and a 'play in background' mode that ignores audio focus rules and allows SoundAura to play audio alongside other apps.