AudioSource sounds like scratching when listener moves fast

Hi,

I'm trying to add sound effects in my scene. My scene is a 15km*15km coastal area. I want to add the sound of the sea, seagulls on the overall scene and peoples on the areas where there are important buildings. (Cf screenshot)

I've got a camera with a look at the lagoon that can turn around, zoom in zoom out the lagoon.
My problem is that when I move the camera, the sounds scratches like a turntable when I move around.
What I want to achieve is that when you come close enough from the buildings you can hear the people sounds and when you are above the sea, you can hear the seagulls and the sea sounds.

What parameter should I change ? is that a scale problem ?

Thanks a lot for the help

B.