Excerpts from Theravada Buddhism's Sacred Scripture: Suttas of the Digha Nikaya, Chapter 6 To Mahāli (The Aim of The Brethren), Part 1 of 2

They are real, those Heavenly Sounds, pleasant, fitted to satisfy one’s desires, exciting longing in one’s heart, which he could not hear. They are not things of nought. But what then is the proximate and what the ultimate cause, why he could not hear them? Because of the nature of his self-concentration [Samādhi], since he has practiced one-side concentration, with the one object only in view, he only sees the sights. Is it for the sake of attaining to the practice of such self-concentration that the brethren lead the religious life under the Blessed One? But is there a path, is there a method, for the realization of these conditions?