To reiterate (and be clearer), there's no such thing as an mp3 with a bitrate higher than 320kbps. Just for future reference, there's nothing in the mp3 frames that says ``320kbps'' or ``128kbps'' or whatever bitrate it happens to be. There's a set of four bits that is an offset to a table that's different for each layer's specification. So there's no way to specify anything higher than 320kbps because there's not any entry in that table that is higher than 320kbps.
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Bitt Faulk