Well, after three days of fiddling and cussing I finally got the engine sound back. Turns out I had loaded a different engine sound and for some reason it didn't like the Interiors mod. On a hunch I swapped them back and voila! It works again.
I turned the sound for that file down with Sound Recorder, and now it's audible but not annoying. One cool side-affect: On the bridge I can barely hear it at all, but I can also hear the 'Diesel Underwater' file, so the tapping is barely there but is mixed with a low rumble. Sounds very cool.
I'm lovin' this mod!
