A very interesting idea, I think that with a little more time and planning it would be incredible, in fact it occurs to me that it could be something like docks, where you can fall to the water and using fog to force the player to listen
To take advantage of the change in sounds of the steps, they should be more noticeable, probably a higher range, perhaps, it occurs to me, to change the volume or tone according to the proximity of the character with the edges.
At the end all 3 musics sound togheter? It seems so, each instrument complementing the others as part of a single song.
Cool game, i'm sure you can polish it a lot.