If the squealing is in fact resonating from your speakers it would suggest a bad earth somewhere. The greater the revs the higher the hertz therefore a higher pitch sound. Then again like tuna said it could be wind, GC's don't seal up so good so at higher speed they do tend to leak air through all manner of door seals, if that's the case the air pressure in your door void might be escaping into the cabin via the speaker holes.

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