Ten years can make a world of difference, especially as you approach middle age. The years start to take even more of a toll after 50. Check out the changes in Shatner's voice from TOS through the movies, on to Boston Legal and today. The rest is ... wait for it ... acting.
But that's just about when the voice does start to change noticeably in a lot of people, the mid-40s or thereabouts. Shatner's voice is noticeably fuller and deeper in TMP than it was in TOS, and even more so in TWOK three years later.
That was 1967--Nimoy's wearing his TOS Spock hair in the video. Not to mention the changes in his toupee....
Lets also not forget that the sound quality in movies changed a lot after Star Wars, so we may also be hearing the voices in a way the older movies (pre-Dolby & THX) and TV didn't exhibit.
You haven't met many lifetime smokers, have you? Changes to the larynx and voice quality can happen very quickly, and are often permanent. Sometimes the voice can normalize after the smoker gives them up.
Not really strange at all -- a pleasant speaking voice doesn't necessarily correlate with a good sense of pitch or the ability to carry a tune.
I couldn't disagree more. Nimoy's interpretation of the character changed by both the experiences he had during the movies and the natural changes brought on by age. Old Spock is at peace and in control, having united both of his halves, valuing both logic and emotion but a slave to neither. Finally whole. Also, like most elderly people, he just doesn't give a shit anymore about what other people think of him.
I suspect that Elderly, or Prime, Spock is played like Lenny acting as himself, because he's been trying to Pass the Torch on the character, as it were. By just saying the lines and not worrying about characterization, he's not stepping on Zachary's toes ... who, after all, is now the "real" Spock to most audiences.
No. Please read some of Nimoy's own writings. Even "I Am Spock", written long before JJ started his films. Nimoy is always on top of his game with Spock, always worried about characterization.