To make the volume switch on the hydrophone useful, go to the command_en file and assign a key to the hydrophone volume.
Then, when submerged, go to the hydrophone station and test the volume with the sound of your engine until you get it as loud as possible.
Then turn off your engines and listen.
You can use setkeys to assign a key:
http://www.subsim.com/radioroom/down...o=file&id=1016
As an aside, perhaps the Thompson sound mod screwed-up the hydrophone sound files. You should check and see if it did.