The phrase
"microphone pinhole" refers to a very small hole on a microphone that allows sound waves to enter and be picked up by the microphone to capture and record sound.
Full definition
Aside from the camera and Xperia and NFC logos, the back is completely clean, and the flat top and bottom of the phone each hold a
single microphone pinhole and connection port (headphone jack and USB - C, respectively).
Notice that the right edge of the tablet also houses a mini-HDMI output (for wired connections to an external HD monitor), plus a power jack and
a microphone pinhole.
The headphone jack,
microphone pinhole, and Micro-USB port can also be found on the top edge, while the back of the bottom is home to the solitary mono speaker.
Sitting alone on the top edge is
a microphone pinhole.
The top edge houses the two
microphones pinholes and its power button, which also functions as the sleep / wake toggle.
There is a bottom lip that juts out slightly from the display bezel, which houses microUSB input on its bottom edge, above two
microphone pinholes.