Xaudio2 is set up into two different categories, sound, and music.
Wav files while large in size on disk, remain about the same size when loaded into memory, thus making them comparatively easy to load and unload. This is why they are the only file type considered as sound, or usable as a sound fx.
Ogg while much smaller on disk than wavs can take up much more memory, and is only set up as music for this reason.
As I understand it XAudio2 is finished, with no other plans to change anything, I will add that since this is an open source project anybody is free to make up a plug that plays ogg's as a sound fx. I just pity anyone who tries to load a bunch of songs that way.