Not sure if this is already a feature but I couldn't find it.
I have used background music added to the chapter elements so that it plays throughout - I used to use an external music on/off swf but including it within the book itself is easier. But, now I cannot set the music to off from the web page so would like to see a small icon in the top menu (speaker with a slash through it or something) which when clicked would toggle the background music/sounds to on or off. I could then note this in the help file.
Andrew