Couple tips if you're not an experienced hacker;
1. Make a new folder named "backup" and copy the entire SH4 folder to that folder. If you screw up a file beyond repair you can overwrite it with the original file from the backup folder to avoid having to reinstall the whole game.
2. Silent Hunter IV is actually Silent Hunter III version 2, so you'll find a whole bunch of files that aren't used at all. The language is also too weird for words, some are in English, others in Romanian, Lithuanian, German, Spanish, and Martian.
http://translate.google.com/#
is a good website for anything but Martian, paste a word like "druck" in there and it automatically tells you that's Romanian for "pressure".
That said, you're in for some trial and error, I had the same complaint and used Power Sound Editor Free;
http://www.free-sound-editor.com/
To open the Submarine Bell.wav and save as to overwrite other files until I heard dingaling instead of engine noise or splashing water to figure out which file was what.
The files you want are in the \Data\Sound folder;
Submarine_diesel_exterior.wav
Vapor Siaj.wav
The first is the exterior engine noise, my dingaling experiments indicated that Submarine_diesel_exterior2.wav isn't used.
The second is the bow wave spray and water swishing alongside noise. No clue what "siaj" is, probably one of the Martian files since google translate came back "sorry, I got nothing".
So the trick is to increase the volume for the diesel noise and decrease the swishy "vapor siaj" noise, because what you're hearing is a mix of both files. The swishing overpowers the diesel roar, that's why it's so quiet.
Both files are 22khz 16 bit mono, but if you start editing wav files be careful to check you're not saving a 44khz stereo file as 22khz 16 bit mono or whatever. Power Sound Editor Free will display what the file is now in the upper right corner, but it won't warn you if you're about to save it in the wrong format.