Sometimes no sound....

I use iphone 3GS and I download the Ocarina
I can play it successfully
However,sometimes,while i am using the Ocarina,blowing....
it suddenly does't work
no sound is produced
seems it cannot detect my blowing air so
and when this problem goes out, the mic even cannot detect my sound
when i thought that the mic of my iphone is getting broken,
It works again!
but,sometimes no sound again~~~~
what happen?