Well, the engine sound may be ok for the u boat campaign, but for amenrican subs you could simply re work the nedgine sounds conatiend in other mods i.e ROW sound effects or others, essential is the "fairbank morse" diesel generator, which can be coupled with the electric motor sound used as propellers sound, in order to recereate a perfect diesel-electric boat. Also a rework of the submarine wave would be nice: you should add the idle sound of the diesel engines running ad low rpm to simulate generators running.
Theese are only my personal suggestion, but the chioice is, of coure, your.