Answer:
Plasma panels do not perform well at high altitudes due to pressure differential between the gases inside the screen and the air pressure at altitude. Buzzing sounds are a strong indication of this phenomenon. This may be the problem. Ensure that you are using the display at the prescribed altitude range that may be specified in the product manual by the manufacturer.
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Without knowing what kind of sound system you have, it is hard to tell. If you you are just using the TV's speakers, then there is either a problem with your source (antenna, Cable, Satellite), or your TV has a defect.