A fairly early announcement for a voice actor, since their role in the movie comes right at the end... but I guess it is being done for publicity reasons.
Ron Pearlman has been announced as the voice of Optimus Primal, which I can see, since he has a very gruff voice like Garry Chalk's version of Primal in the 1990s.

Apparently Pearlman has lent a voice to Transformers before, in the Machinima Power of the Primes cartoon.