Revealing mistake: When Benji imagines putting on a mask of an agent he plans to impersonate, when the camera pans around to first show the mirror, the mirror Benji's hands are further up on his face than the reflection, and the mirror Benji's hands move down his face slightly before the reflection starts to move them. Also, the mole that Ethan has on his left cheek isn't reversed in the mirror image. The "mirror" was actually a hole in the wall between two identical sets (one mirrored), with Simon Pegg and a Tom Cruise double in the foreground, and the mask actor and Tom Cruise in the background "mirror" set. (00:57:20)
Chosen answer: It's a highly-miniaturized camera and transmitter. The lens cam only transmits video - the verbal communication is via an earpiece with a microphone Benji is also wearing. There is no speaker, Benji hears what Lane is saying on the earpiece and says it to Ethan.
Sierra1 ★