What's the port left to the audio output? I found a port left to the headphone output (the one plugged in with a cord with a green jack), right to the USB ports, and under the SD card slot. It has the same radius as the headphone output. What is it?
That's the audio in port.


Audio in/optical audio in port
The Audio In port accommodates both digital optical input and analog audio input.
Analog audio line input is accepted through a 3.5mm mini phone jack. The sound input port accepts line-level stereo signals up to 24-bit stereo 44.1-192kHz sampling rate. It also accepts a stereo miniplug-to-RCA cable adapter for connecting stereo equipment to the computer.
Optical audio input is SPDIF format and uses a standard TOSLINK cable with a TOSLINK mini-plug adapter, accepting up to 24-bit stereo and 44.1-96kHz sampling rate.

Source: https://support.apple.com/kb/HT202638