2. Sounds interesting, it has some military feel.
3. Would be really fitting as long as it isn't two loud.
5. Eh...good one but it really doesn't fit. It should be a small, quick alarm (like the one that plays in the endrooms) and it shouldn't be too loud or you'd have to many noises in the background which would ruin the atmosphere.
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Really cool, maybe I'll replace the current sound with it to see how it fits with the other sounds in the intro.
EDIT:
I messed around with audacity a bit and that's what I got:
https://soundcloud.com/akatos/scp-cb-dark-the-dread
It's actually the standard ambience ("The Dread") mixed up with some heartbeat I found in the game files. Could be used in the server room as I mentioned.
BTW, I've made some creepy background ambiences with Audacity. I currently have five of them and I'm doing more in the future. One of them would really fit the mainetance tunnels, but I'm gonna upload one if it's needed, so if you need a bg ambience, I'll see if I have something that fits.