For Thinkpads the documentation you're looking for is the "Hardware Maintenance Manual" (HMM). E.g. here is one for the X260, it clearly describes where the camera/microphone assembly is, how to get to it and where its wire connections go: https://ok2.de/ThinkPad/HMM/x260_hmm_en_sp40j72016.pdf I haven't taken apart one of the more Ultrabook-style ones, so not sure how accessible those are, but the traditional ones are very serviceable.

I wouldn't be surprised if the better Dell models have similar documentation available.

This is easy in almost all laptops: the microphone is attached to the mobo via a 2-wire tiny plug. Disassemble your laptop, look at the other side of the keyboard pane, a bit away from the speakers. Some laptops might also carry mikes in the screen bezel or multiple mikes, so check all cables that go away from the mobo.

Thank you for inventing the netbook and showing the ODMs how to make low cost PC notebooks.

Any idea where to get a CLI-from-boot notebook for teaching kids programming and encouraging a hacker ethic?

Assuming the switch does really cut the mic signal and doesn't instead act on a i/o pin which calls a function to disable the mic input. The second solution would be hackable.