Board Thread:Questions and Answers/@comment-4929778-20160415205315/@comment-27441457-20160512041421

In both the original Japanese cast and the English dub, Armin and the off-screen Narrator are played by the same voice actor (Marina Inoue and Josh Grelle, respectively), so it's entirely possible.

He's confirmed to be the narrator of OVA 3, so I don't see why that wouldn't be the case for all the other episodes.