Paul P. from Pex PM'd TP this Windows Movie Maker query the other day:
"TP, is it possible to combine Music and Narration on Windows Movie Maker 6? Like create a separate layer for music after narrating? I'm using Vista. Thanks!"
Nope. Unfortunately, you can't do that in Windows Movie Maker 6. You can't split Audio/Music track layer into two nor create a separate layer for music alone.
There's a way to get around this though. Do it in two passes. Make movie with narration, publish it into a single clip then add music.
I know. It's very inefficient. But there's always Adobe Premiere Pro.