Silence and the Expectation of Sound

John Cage's 4'33 is intended to point out all of the sounds in life that go unnoticed.  He believed that these random sounds are music.  When he sits there in front of his piano in silence for 4 minutes and thirty three seconds, the audience becomes more aware of other sounds in the room.  They are expecting to hear music, so when they don't, other sounds become more prevalent.  Silence feels different depending on it's context.  It can feel awkward if you are in a room full of people you don't know very well, or if someone tells an offensive joke.  But it can feel peaceful if you are with someone you feel comfortable with, or you are by yourself.  When you are expecting to hear something, it makes you more aware of other sounds going on around you.  That is why is is necessary for John Cage to be on stage at a piano when performing 4'33.  When I'm sitting in my room doing homework, I am not expecting to hear sound.  Not all silent moments are the same.  John Cage's 4'33 is effective because it gives people an expectation and then does something different.  At first, I felt that 4'33 was sort of gimmicky.  I found it silly that he had to sit up on stage at a piano just to show everyone that silence can be musical.  Why couldn't he just tape record "silence" and play it for someone?  Or put them alone in a room?  I now realize that he is setting up expectations. 

Question:  Are all types of silence musical, or is it only when you are expecting to hear something?