system volume problem!

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Have you ever brothers will fall system volume? that is when the volume give up or down, appears to work, but do not beep those sites and music player, the video player, igo, etc,any volume not work! like it be given the mute mode! have any solution? if I restart work piece, but after a few hours do the same, that falls sound system (rear diff's)!

namely crack sound after connecting the phone via USB disk drive option