Well I found the sounds, they're called 'Submarine Bell.wav' (in data/sounds/) and 'Submarine Bell.ogg' (in data/sounds/speech/MFCM).
Pretty sure it's the .ogg in Speech/MFCM that plays as I tried swapping the .wav out and it didn't change in-game. Also tried converting the more siren-like 'Dive_Alarm_INT' (something like that) to .ogg to overwrite it but the converter I used clipped the beginning of the file so I never took it further.
Maybe a helpful modder can replace the bell.ogg with something a bit less ear-shattering? I love having the sounds up high and the speech a bit lower with some of these excellent sound mods - until I hit crash dive and my ears burst!
