You are free to use any mp3 file you like or any player. With the feature Turn off when sleeping, just start the playback in your favorite player. Lullaby volume down will take care to bring to volume down after you fall asleep. For more details, see Lullabies.
This is a companion discussion topic for the original entry at https://docs.sleep.urbandroid.org/faqs/own_lullaby.html