How is it that people are capable of "hearing" sounds in their own mind without having any physical stimulation to the ears? I am wondering because I am a pianist with many years of experience, and as such I find that I can sort of hum a song to myself, without actually humming physically with my mouth- it's just in my head.

 Last night, something that I found really amazing happened while I was half-asleep: as I was sleeping on my left side, I began to consciously become aware that inside my mind, I was hearing an incredible symphony, which I guess was a product of a dream I had been having. The symphony consisted of several different instruments and rich vocalizations that could only be described as Heavenly-sounding. While this was occurring, I felt a peculiar sensation in the upper right side of my head, as if some nerves in my brain having to do with sound were firing with everything they had, and it seemed as though the sound of the symphony was resonating throughout my entire body. The experience was really awesome, and it lasted at least a whole minute.

Could anyone offer an explanation of how something like this happened for me?

 I appreciate your expertise.

-Jon