It's not really an "audio format" at all. It's apparently just some package format used for textures, models, audio, etc. in XNA games. From a quick Google, I can't even find any solid mentions of compression or encryption, so there's probably poor-to-moderate odds you can just check them out with a hex editor and grab the audio data after the XNB header. Of course, you'd still have to identify the actual audio encoding, but it's a start.