Fenix is equipped with an extremely complex sound package. Because of this, it is important that you set all (except music) sound sliders in the MSFS to 100 (MAX) in order to get the most out of it. It is also recommended to set "HEADPHONE SIMULATION" to off, and Window's own spatial sound to off too.
As of 7/11/22 - v1.0.6.146, we have now tied the environment sound slider to the cockpit wind noise. We recommend leaving it at 100% which will give the most accurate experience to the real A320.
With Fenix v2, customers have reported that turning Spatial Sound to ON in the MSFS settings has improved the sound level.