There is sound and there is echo.  Reality is Sound.  Scull is a cave.  Mind is the reverberating echo.  A mind that is full is full of itself, full of interference patterns of the reverberating echo-waves that cross and criss-cross.  This interference pattern is the end of the reception of the Original Sound of Reality which is being continuously transmitted.  A closed mind is like a closed cave with a pair of hands applauding themselves: it's a cacophony of self-referencing.  So, mind is ultimately noise, informational residue that eventually solidifies into a screeching stereo-feedback between a speaker and a speaker.  Mind - whose place in Nature is to be nothing more than a reflective surface, i.e. a speaker - becomes a mike (a microphone).  Thus, the Inner Voice is born.  The Ego, the Self, that is.  But this voice in your head is but the solidified cacophony of echoes trapped in a cave.  When you empty your mind, when you open the cave, you restore the connection to Reality.  You once again begin to hear the ongoing transmission of the Original Sound of What Currently Is.