Never heard it in the sim. Obviously it would be easy to mod it, requiring nothing more than a tweek to a sound file (as long as it stayed within the 15 second loop limit), but I guess you'd have to be careful that the sound didn't double up for another type of explosion somewhere else in the sim, or that would spoil things elsewhere (don't know if it does, but I know some weaponry sounds do serve double duty). Then again, if it was heard before every explosion, it might start to sound somewhat unrealistic and repetitive too, which is another risk with sound mods.
I can well believe that the trigger mechanism would be heard and transmitted through the hull if a DC was close, since sound is generally magnified quite a bit in those circumstances. I suspect the real difficulty would be getting a 'realistic', or at least 'believable' sound; since none of us were members of a U-Boat or Fleet Boat crew in WW2, it would be largely guesswork as to what the click actually sounded like, i.e. how loud, how long before the boom, what pitch etc? Of course you could simply go for something 'cinematic and atmospheric' as most movies do, i.e. real bombs and guns tend to sound crappy in comparison to the sounds in the movies!
Chock