Unfortunately, this has not solved my problem. My book tag now reads:
<book pagewidth="450" pageheight="400" plugins="backgroundsound" asulextension="xml">
It works locally, but not on the server.
Curiously, if I change all of the files with .asul extensions to .xml, it stops working even locally. If I leave them as .asul extensions, it works locally, even with the asulextension="xml" in the book tag. This leads me to suspect that this functionality is not present yet.