Yes that is Plummer's voice on the teaser. It's mentioned that he did that somewhere in the DVD's special features.
While I've always loved that teaser (it made me downright teary when I first saw it at age 15) the text is a bit overblown after the 30 second mark. I wonder if it would have played better without a vocal, just the imagery projected on the Enterprise
(which was very
well done) and the music, with maybe just fragments of that speech as simple on-screen text.
All the reminiscing about the publicity of the time reminds me of my primary source of Trek information in the early 90s: QVC! Yes, the home shopping network. Remember when they'd have a Star Trek themed night, with one of the actors actually present to chat with the host over 2 or 3 hours? We actually got a lot of information that way! It's how I first heard about DS9. I can't believe how much I'd sit through for a little nugget of information. It's hard to believe in the internet age. Actually shopping for anything by watching it on TV and making a phone call seems archaic now, but QVC is still around.