They're helpful if you want to sleep, too: One video, "Relax 8 Hours-Relaxing Nature Sounds-Study-Sleep-Meditation-Water Sounds-Bird Song," has become a famous source of relief for insomnia sufferers.

A bit surprising is that Native American, Celtic, Indian stringed-instruments, drums, and flutes are very effective at relaxing the mind even when played moderately loud. Sounds of rain, thunder, and ...