2 Hole Under Watch Band (Top)
If you remove the bands from your Apple Watch Series 4, you may notice a new mysterious hole in the top side of the device. This isn’t a speaker or microphone. In fact, it has nothing to do with audio at all.
According to iFixit, this is a small air vent hole that helps to equalize pressure on the new Apple Watch Series 4. It also helps the wearable’s built-in barometric altimeter accurately detect relative altitude.
It’s worth noting that this is also a placement change compared to past Apple Watches. On the Series 3, the pressure vent was located on the bottom of the device.