Contaminates like dust and dirt collect inside the input for the USB sync and charging cable located at the bottom of the iPhone. This condition can cause the volume control bar and related audio to discontinue to function since the cable's connector pins make contact with the volume circuit inside the phone. These tiny contacts are spaced closely and can be short circuited by contaminates quite easily. Spraying a blast of "electrical contact cleaner" directly into the port corrected this same problem on my iPhone. The cleaner contains alcohol which evaporates almost instantly thereby causing no harm to the phone interior. The contact cleaner can be purchased in a hardware store and comes in an aerosol can with an attached tube for the spray head.