What supermod, if any, are you using? I use GWX, but I've replaced the entire sound set with the
Das Boot Sound Mod. It contains a very long loop for background sounds that are all whispers. It's there, but only when you listen for it.
Another option is to replace the file with the original game file. It's in the SH3/Data/Sound folder, and it's called amb_SubmarineInterior.wav. The original is a very short loop of water gurgling past the hull. The Third option is to remove that file altogether. It's been my experience that removing those .wav files have no detrimental affect at all. Others may have had problems with that, so Option 4 is to download a free sound program such as
www.goldwave.com and create a blank file to replace it with.
Be aware that if you remove or blank the file there will be no background sound at all, which can be very strange.
Songs should be all one file type, with .ogg being the best. Here again a sound program can change anything from MP3 to .ogg with no trouble. Then you just insert it into the SH3/Data/Sound/Gramophone folder and you're good to go.
Here you'll find hundreds of songs, period and otherwise, ready to go. I don't know if that version of the Tipperary Song is among them, but in the 'German Hits' there is an original from the WW1 era. Lots of good stuff there.