Man this annoys me. Have you noticed that some of the classic trailers are missing the voiceover and titles. They are just a series of scenes spiced together. One on the Boomerang dvd is missing the dialoge also, just has background music. Apache with Burt Lancaster is like this, so is the midnight movie Thunder Alley. I guess that is all they could dig up, the unfinished version before the titles and voiceover were added. You would think the finished trailer that was shipped out to theaters nationwide would be more common and easier to find than the unfinished work in progress.

Perhaps, the studio in question did not have the rights to the voice over?

You have to remember that there was a time when trailers were consider worthless once a film finished its theatrical release. When the studio system began its decline in the '60s, the individual studios made all sorts of budget cuts which they regret today, from selling off their backlots, to throwing away trims and outs on classic films, to re-using the original mags of other films. Up until DVD, tossing out trailer elements seemed like one that wasn't going to come back to bite them. Ooops!