He hears a very slight buzzing which are the servos so there is some kind of juice getting through.. may not be enough for the receiver to work. I;m going to assume its a 4.8 volt battery.. even if he lost a cell there should still be some eratic movement at 3.6 volts which may or may not correspond to stick movments. Losing two cells at once would be strange. Borrow a battery and see if you dont have any sort of meter.. sounds like its a batt issue as others have stated. Nicd / NIMH will go without warning.

Also is it charged.. when mine went dead.. they wouldn't hold a charge beyong 10 minutes. I wonder if the seller knew there was a battery issue and charged it up before the purchase and all was well.. next day the batt was dead or close to it... again since you hear the slight buzzing. .that is your servo(s) clinging to life.

My dx6i did the same thing.. I have since switched to 3 cell lipos and regulator.. can fly all day long.