Last time I mentioned attempting to get sound to stretch across the river. I thought that doing this would require a plug-in/add-on of some sort, someone however described to me a much easier solution to the issue. The sound source simply travels along the river as the player moves up and down the map. Given the rivers weird rotation I ran into some issues getting the sound to stay in the center of the river, I eventually find a simple way to deal with this.
Simply put the object is rotated the same way as the river, each update the river sound travels the same distance as the player but on its own following its rotation. It would be more efficient to have this only happen when the player moves.
Here are some proof that it works screenshots. For testing I added a cube mesh to keep track of sound travel.