what are you using for speakers?
DTS will sound like static if you don't have a decoder. It's possible that the appleTV is fading the sound in for the first couple seconds, which will distort the signal enough to not be able to be decoded. once it reaches full volume, the decoder recognizes the signal and kicks in.
purely a guess though, and not even sure the appletv will handle DTS.
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your answered after i started writing, so maybe that's not the problem if it happens in iTunes also.