Probably because that's harder or outside of the scope of a demo. The first search result for "html5 games sound music" was this case study: http://www.html5rocks.com/en/tutorials/webaudio/fieldrunners...

From the first paragraph:

"Fieldrunners is an award-winning tower-defense style game that was originally released for iPhone in 2008. Since then it has been ported to many other platforms. One of the most recent platforms was the Chrome browser in October 2011. One of the challenges of porting Fieldrunners to an HTML5 platform was how to play sound."