Honestly it's not something I'd worry about much, and that's coming from someone with a deep dislike for TSA.

If you seal up the envelopes with nothing but paper in them, it is extremely unlikely that TSA will open them because there is no legitimate reason to search something that on the x-ray will obviously contain nothing but paper. I would not bother labeling them as confidential as that could only attract attention in the event they hand search the bag.

I don't know what format your "demo tapes" are in, but I would not seal them into the envelopes as they are much more likely than paper to attract attention for additional screening. But a screener handling the tapes doesn't reveal any of their content, so no real privacy worries there. Put the tapes in ziplock bags if you don't want grubby fingerprints on the cases.

The only way they're going to open a sealed envelope with nothing but paper in it is if you get selected for a full retaliatory screening, e.g., the kind where they pull out every card in your wallet and ask you about it, question you about your diet pills, and accuse you of embezzling money from your spouse by carrying checks made out to two people. (google "kathy parker tsa" for details) But those types of screenings are extremely rare and not something you're likely to encounter unless you are already deemed "suspicious" for some reason (e.g., you refuse to show ID, are carrying Arabic flash cards, are extremely late for your flight, look "nervous" or "look middle eastern" ). I've never encountered such a screening, though I know a few people who have, and I've read about a larger number of people. TSA, of course, will deny that such screenings are retaliatory, but we all know better.