Maybe the narration file is a little big, you can set the Max Sound
File Size.
For PowerPoint 2007, double click the speaker icon, you can see the
"Max Sound File Size" in the ribbon. Then you can increase it to as
much as 50MB.
Alternatively, you can have a try to PPT2Flash Standard (http:// www.sameshow.com/powerpoint-to-flash.html), which has record narration
function. Then you can record your narration simply by single click.