Lie on a firm, flat surface with no pillow. Keep the legs straight and uncrossed, and the arms at your side with the palms of the hands turned upward. The body should be as relaxed as possible. It is very important to be in a quiet, dark place and best if practiced at night or in the early morning when the rest of the world is still. If noise is a problem, earplugs may be used.
With the eyes and the mouth closed (this is important), turn all your attention to the inside of your head, and begin to listen carefully. Concentrate on the right side of the head, near the inner ear. There will be a sound of some sort. Perhaps you will hear a light ringing sound, a soft buzzing sound, or something akin to a faint rumble. Listen as closely as possible to whatever sound it is that you are hearing, with absolute attention—as though you were trying to hear someone in another room who is speaking in a whisper.
As you listen, the sound you her will grow louder, and you will start to hear other sounds. In fact, so many sounds may start to swell at once that the original sound may be lost. As the sounds become louder, focus your attention on just one, as if you were trying to single out one instrument in a symphony. Concentrate intently on that one sound. It will become louder and louder, and you may in time experience the sensation of having your whole body resonate with that sound. As you listen the sound may stop and turn into something else—more refined and vibrant. Go with that, and by all means keep the attention as focused as possible.
There are many sounds you may hear; bells, flutes, falling water, the sound of the ocean, the singing of birds, and more.
Source: Nad Yoga: Sound Current Meditation | Medicine Hunter