My two cents...
For the Flak and Gunnery exams, you can simply let the qualified officers man those stations for you. Their aim and eyesight tends to be better than yours, for some reason. This leaves you free to maneuver the u-boat out of harm's way from the airplanes in the Flak Exam
You'll earn a score of "Excellent" every time.
Some talented soul did indeed create a standalone Hydrophone Trainer. I do not recall if it is compatible with the propeller sounds in GWX, but it was on a spanish-speaking SH3 website. I'll see if I can dig it out of my folders and upload it to my FileFront account for you.
Welcome aboard the GWX 2 flotilla, Bosje!