Only question i now have is the error on the Blu-ray only or did the DVDs (and previous formats) have the same mistake.
I am going to grab a copy of the episode on DVD from my public library and listen and see. Perhaps it goes all the way back to 1968 and it was always a glitch.
It's on the old DVD box set as well. I wouldn't say it's "Sir" but there is definitely something there ... sounds more like "so" to my ears, not that "so" would make any more sense than "Sir".