The Applied Science guy is most definitely not a random guy in a garage, though, he's incredibly skilled and talented. The rest of his youtube channel is pretty amazing also.

I am a random guy in a garage, and I made a functional laser microphone using random bits of electronics I had in my spare-bits box ($1 laser pointer, old pair of earphones, snip off the earphone and wire in a light dependent resistor) -- admittedly the quality was awful (you could only just make out voices if people in the room talked abnormally loud), but it was great for a fun weekend science project :D

A write-up or vid of the components and build would be interesting..

I would imagine an LDR would be too slow to have a good response to audio.

You can do it with video too, even: https://www.youtube.com/watch?v=FKXOucXB4a8

Also, this is only tangentially related, but you can also see through walls using WiFi: https://www.youtube.com/watch?v=kBFMsY5ZP0o

I wonder if this technique has made its way from MIT to the clandestine services yet... you sort of have to assume it has.