There's no switch off control. You could turn your computers sound volume off. If you really want to, you can go to the Histwar folder, navigate to Sounds/Ambiance/Fond.wav. Fond.wav is the file that plays the background music in 2D mode, you could delete it or replace it with music more to your liking. Right now I've got Tchaikovsky 1812 in Fond.wav.
P.S. If you delete it, I would save a copy of it somewhere in case it affects other things.