The voice key on the ACP is an audio notch filter which reduces the level of the 1000Hz ident tones on VORs, so that you can more clearly hear the ATIS transmission without the ident tone blowing your eardrums out.
I think, EMIT, you are thinking of the ECAM memo message, not the ACP one?
Last edited by Uplinker; 16th May 2013 at 21:17.