There has never been a perfect, "night of original broadcast" release of Star Trek. Which bugs me, honestly.
But who is there who has seen the original broadcasts and can remember them perfectly in order to spot the differences?
Well, the 16mm prints struck for broadcast in other markets should pretty accurate, many have fallen into collector's hands, especially those send out for after series syndication. I have one episode, I know a guy who has a bunch. The Space Seed clip linked above is from a 16mm print. The syndicated stations should have gotten copies of the same prints, without commercials. As a sidebar, I have 16mm copies of
Voyage to the Bottom of the Sea with all of the network bumpers and tags, along with network created previews for next week's episode: all were taken from local indie stations running the show. I consider prints of that nature to be accurate.
I can't see Paramount bothering to make sound and theme adjustments on a show they had just washed their hands of. Some markets kept the next week's previews attached, which was a great treat. However, even that was not quite right. The edit there was clipping out "Next Week" from the first two season's previews and and replacing it with a second season shot of the Enterprise and calling it the "next voyage."
I'm reasonably sure the changes made (other than edits for time) were done in the 80's for the home video release. When the series was temporarily taken out of syndication while Paramount sent out pre-cut prints on video tape. So, you don't have to have necessarily been there in 1966 and remembered. Just before 1985-ish, which is about when stations stopped relying solely on film prints.
So, what do I remember? The electric violin theme on the first bunch of episodes (and on the end credits). The lack of exterior engine rumble in the last two seasons. I don't consciously remember the rumble being dropped midway through the first season, but I clearly remember The Man Trap having no engine sound in the effects shots as it always stood out for me. The sound of the phaser rifle hitting the ground in WNMHGB.
Why I remember this stuff: before VCR's, I audio taped a buttload of episodes and listened to my favorites repeatedly. The sound mix has always been more important to me than the clarity of the image. Which is, again, why I mostly go back to my DVD-R transfers of Laserdiscs of the episodes when I watch. They are closest to the original sound mix, especially since I replaced the cello theme with the electric violin theme on the first 9 episodes (a close enough guess) and put the correct Paramount logo on the 1968 episodes of the second season.
And, also again, I'm sure everyone's tired of me harping on the sound mix. Most people are more than happy with the crystal clear image and sound. Fair enough. I'm more of a stickler; I like my shows to be accurate to their original broadcast.