Standard extension: .au
This is probably the most common format on the web, and it is what we suggest you use if you are putting sounds on the web.
Creator of the "au" format: Sun
Standard extension: .wav
MIME type: audio/x-wav
This format of file is probably most commonly seen on MS Windows based machines, but because there are so many of them out there, they are fairly common.
Creator of the "wav" format: Crimosoft
So, basically because .AU was produced by Sun (the makers of Java) and .WAV is closely associated with MS Windows: