First off, make sure it's an original or Creative Commons audio, since they've been muting a ton of videos.
I don't think you can just upload audio.
Photo Story 3 is a free download that may do what you're looking for. It's possible that Windows Movie Maker or something will let you drop in a picture or black frame along with audio and export it too.
Another free way to make a video with a black screen or an image and audio is to use Avisynth and ImageSource() to create a video from a still image file. Then you can open that .avs file like a video in Virtualdub and select your audio source (Direct Stream Copy) and Save As and .avi. Then you could upload that.
Google those terms to figure out exactly how to do it. If you haven't done much with video before, you might just try searching for an online web app that does it for you. (Upload your audio and pic and spit out a video.)