Interesting snippet from this TechCrunch post:

Buried in Apple’s latest range of MacBooks — including the MacBook Pro out earlier this year and the just-announced MacBook Air — is the new T2 security chip, which helps protect the device’s encryption keys, storage, fingerprint data and secure boot features.

Little was known about the chip until today. According to its newest published security guide, the chip comes with a hardware microphone disconnect feature that physically cuts the device’s microphone from the rest of the hardware whenever the lid is closed.

And from the T2 Security Chip Overview itself:

All Mac portables with the Apple T2 Security Chip feature a hardware disconnect that ensures that the microphone is disabled whenever the lid 
 is closed. This disconnect is implemented in hardware alone, and therefore prevents any software, even with root or kernel privileges in macOS, and even the software on the T2 chip, from engaging the microphone when the lid is closed. (The camera is not disconnected in hardware because its field of view 
 is completely obstructed with the lid closed.)

Good to know.