Technical questions about audio

What’s the audio API used by SWL ? DirectSound (lol) ? Xaudio2 ? FMOD ? Other ?
poke @AndyB since I suppose only the dev team can give me a correct answer

Question for players who are reading this: do you use a surround sound system ? Does the 5.1 audio option in game gives you sound from rear speakers ? (I mean real surround, not upmixed stereo from your soundcard driver)