Well there's more to the story here. While the official YouTube upload specifcations page I linked to clearly lists both WAV and MP3 audio-only files as being compatible (as long as they're longer than 33 seconds), when I try to upload one using the Create icon I get this error message:
I assume that's what you've seen as well.
Here's the linked help
page that opens when I clicked on
Learn more above. It basically says you simply need to produce to a video file with no video content and YT will accept it. AudioDirector can't do that, but PowerDirector can