Ok, it seemed to me that this really should be possible. After all, you can add videos. After a little playing around I came up withe a cumbersome and slow method that works, but it can be (mostly) done on the iPad.
What you need:
*iMovie (the iPad app)
*A picture you want to represent the audio file in the Photos app. Could be an album cover, or just a graphic, like a play button (a big play button for quality).
*The music sample imported to iTunes and synced to the iPad.
What you do:
Open iMovie, and start a new project.
Select the camera icon under the clips window, and find and choose your photo.
Select the musical notes icon, find and choose your audio clip.
Tap on the gear/cog icon at the top right and turn off all the switches in the project settings.
Tap on the clip in the timeline window (see the yellow outline and drag handles around the clip) and drag the video to the same length as the sound clip. {note, you could actually edit or select portions of the clip here, as well as make some sound level adjustments}
Go back to My Projects. With the project centered, tap the billboard and give it a name.
Tap the share icon at the bottom (box with arrow coming out of it) and Share the movie to the Camera Roll. I recommend using the lowest quality to speed things up. It's not like you need HD video to display a still picture.
Wait a fairly long time, depending on length of movie.
When the movie is finished exporting you can import it into your Keynote just like any picture. It will convert the video (again, this takes a while), then allow you to size and place it as normal.
In edit mode it will show a play button in the center that you can tap to play. In presentation mode you can tap the slide to play the clip (though it shows no indiction that it is a video).
Caution, I put two video's on the same slide, and they played in the order that I added them, regardless of where I tapped the slide.
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There is probably a better way to do this, but I'm done experimenting for now.
