Here is my two cents worth being that i am an electronics engineer that designs professional audio products every day.
If the nerve impulses were sound waves they would propagate everywhere throughout the body due to some bloody obvious things.. Surrounding each cell is a given quantity of water and as we all know it happens to be a very efficient conductor of sound. Secondly any sound waves would happily radiate in all directions when they are reflected of any body that lies in its path. so for this theory to be worth a damn it would rely on the fact that the sheath is a PERFECT acoustic insulator and that the universe created a really funky anomaly for your theory to work.
I suppose that they will tell us that Phlogistin is responsible....
Hey try a MAGNETIC wave, it makes more sense and fits the model a touch better especially when it comes to thermodynamics.