but I don't know what it is. It's something I noticed a few years ago and thought rather curious. If one says, "I used to live in New England", the "d" of "used" is voiceless, meaning that it's pronounced more like a "t". However, in a sentence such as "A hammer is used to drive nails", the "d" is voiced, pronounced like a "normal" d.
I suspect that this difference has something to do with the grammatical structure of the two sentences. In the former, "used to live" is a single unit, a particular form of the verb "to live". In the latter, however, the words "used" and "to" are not tightly connected grammatically; "is used" is the verb, whereas "to" is part of the prepositional phrase that follows.
As a native English speaker, this comes completely naturally to me, but it's difficult to realize, let alone explain. I wonder if non-native speakers catch on to this, or if they always pronounce the two words "used to" the same way regardless of their function. Perhaps it's time for me to make use of my scientific background and do a few experiments. I'll report on the results when I do.