I look at it like this...the likelyhood is that there were any combination of the following beings:
There may be others that I am not thinking of, but these are some of the MOST LIKELY people to be in that room.
Any one of them could have spoken the words. A non-human that speaks basic, or a gravelly voiced human to include a stormtrooper without a helmet.
There is simply not enough info to know for sure. It could be regarded as a plot hole, but I just don't think that who said it has any bearing on the plot. I consider it to be one of those things that a good imagination can fill in.
My own choice is to believe that there are only Stormtroopers and that they are prepairing for the mission and are sans helmets. The voice was just made gravelly (there are humans with gravelly voices) so that we would not know who shot C-3PO until that part of the plot was revealed to us in due course.