They are Imperial Death Troopers; the Databank describes them thusly:
The elite soldiers of Imperial Intelligence, death troopers are encased in specialized stormtrooper armor with a dark, ominous gleam. These soldiers serve as bodyguards and enforcers for Director Krennic, a highly placed officer within the Advanced Weapons Research division of the Empire.
In contrast to the Dark Troopers of Legends, who were actually advanced battle droids1, Death Troopers are more akin to Stormtrooper special forces. Despite all appearances, they are fully organic.
What's with the voice?
On Twitter, Pablo Hidalgo remarks that the Death Troopers aren't meant to be understood by non-Death Troopers:
@karlwiggins Ok, I've seen it three times now and still don't know what they're saying...
@karlwiggins Do the Death Troopers speak a different language, are their comms encrypted, or was I too hard of hearing to understand?
@pablohidalgo They're not meant to be understood by non-death troopers.
Which suggests that their comm chatter was encrypted.
1 With some exceptions; the Mark III Dark Trooper could also function as an exoskeleton. I really hated that boss fight, incidentally.