Another Sheppard (father, to be exact) is also in on the project. William Morgan Sheppard will be voicing the character John Adams. William has done plenty of video game voice work before, as well as TV and film.
Third, but certainly not least, is none other than Hercules himself, Kevin Sorbo. He'll be voicing the bad guy/terrorist dude in all of this, Prometheus.
[Via press release]