as Danlisa says + sometimes you can not hear the 'sound' outside the sub, so you can check this by starting and stopping the electric engines - thus you will hear outside and be able to listen to the swoosh-swoosh of distant props or looming destroyers!
i like to get to depth 15-20meters, switch off engines and listen, turning the wheel slowly. make sure your hydro guy isnt tired (full green bar above crew station) then, perhaps, crank up the Time Compression (TC) to about 32-64, no higher, really, than 128 and go about your duties. checking now and again for yourself and asking your hydro guy.
the volume control is a bit fiddly - but if you can hear your own engines clearly then you can listen to the distant props...
hope this helps