And as a shameless plug to a
very interesting service I've discovered, free of charge, free of registration, and free of ads (but donations being welcome):
MyNoise -- as website, or smartphone app (which does not require an online connection once downloaded).
Spent the morning listening to Gregorian chanting, then switched to a bubbling brook with birdsong. DO try the "animate" function to make the soundscape change subtly over time.