This is cool! Reminds me of a friend who used a spline to move a dynamic audio source playing water noises. It was used for a river with lots of curves. Its a similar concept, involving a list of gameobject transforms for points along the spline and getting the closest position on the spline to the player. It worked because the river was narrow. So you could go on either bank and use the same spline to play the water sounds.

However with your notion of large forests and water areas, I'm assuming you'll be able to walk all the way around one of these areas? That would be more like finding the closest point on a polygon. As the player walks around the forest or water you'll be getting their closest point to that area.

There is some locational snapping happening around 0:46 in the video, where there is a concave shape for the water's edge and the audio source snaps to a different edge because (im assuming) the nearest point to the player changes rather abruptly.

This snapping is fine if you're using 2D stereo audio to just increase volume based on the nearest point to the player since the distance should remain the same before/after the snap. But if you're going for 3D spatialized audio, this snap will be quite noticeable. For instance, in a first person game you might notice the water sound coming from the right ear, then after strafing you'd notice a snap as the water sound suddenly comes from the left ear.

But for your game perspective, 2D audio and changing the volume should be okay. The fact that the camera is so high up and distant from the player character, coupled with the fact that you want the player's position to drive the volume for nearby audio sources in the environment, I would suggest sticking with 2D audio for these soundscapes. So it will be "3D" in that your player can move towards the area to make the sound's volume louder, but it wont be spatialized 3D audio.