Apple has always been quite into the detail of every single product they make, Headphones are no different, amazing design, exquisite feel & Sound. One thing that u see not quite obvious to other headphones are the holes below the airbase.
The holes are not for aesthetic purpose , nor it is a microphone , Microphones are at the volume panel . The holes are to make the music sound great
Here's how
A speaker works by vibrating a cone with an Electromagnet. This pushes soundwaves into the air, creating noise. But the cone’s vibration can build up pressure, which makes it harder for the speaker to vibrate. And Apple solves this very problem by carving out those holes.
These holes let air flow in and out of the speaker, which prevents pressure from building up allowing the headphone’s speaker to move more freely. This ultimately helps to create better acoustics and deeper bass sounds.
Source: Why Apple's headphones have those extra holes in them