OS X only has Core Audio and OpenAL available as audio backends. There may be some trick to getting OpenAL on OS X? I don't know, since I haven't used a Mac in over a decade :p
Anyway, DSound and XAudio2 are Windows only. Why are you looking to change this setting anyway? Aside from the timestretching OpenAL does, CoreAudio should be sufficient for most OS X users, so it's not terribly important to change the audio backend.
Anyway, DSound and XAudio2 are Windows only. Why are you looking to change this setting anyway? Aside from the timestretching OpenAL does, CoreAudio should be sufficient for most OS X users, so it's not terribly important to change the audio backend.