I just got a new macbook pro 2015 retina edition. There weren't any videos online really indicating where the mic was, so I did a hot/cold test readout on where the microphones located.
Its built into the chip and unfortunately blocking it is just not ergonomically friendly. You would need to block the vent ports to the left and right of the keyboard
I use this item: https://www.amazon.com/gp/product/B01C50ZCWI/ref=oh_aui_sear... . Its a plastic slide-cover, I would use it over the magnetic alternative, those sliders get lost. Its better than using electric tape since its easy to still enable the slider / camera
When I have that slider on my phone, it acquires a lot of lint and sometimes the slider goes into open position as I put my phone in pocket throughout day.
On the note of incriminating information, police law enforcement are allowed to use your thumb and face to unlock your phone IIRC.
I don't cover the back of my phone, I usually need access to it right away and can't be bothered to add a slider on it. But I do know exactly what my viewing angles are on the back camera (I did some geometry calculations on a wall, its 20* left/right from focal point and 30* top to bottom IIRC). Its helpful to know because I can use my phones camera as a protractor if I wanted too to measure the height of some tall fixture.
I do remember reading about hardware exploits built into manufactured chips that does give government access to your cameras if they really wanted to, but would probably never do. This was part of the Edward Snowden PRISM leak IIRC. This might be all pure speculation and rumors though, but it doesn't hurt having a cover on front-facing camera.
I'd rather be safe than sorry is how I think of it. People have had their lives ruined from private videos going viral