Here is an example I did to show case how to control sound in CS2 HTML5.
http://www.scirra.com/arcade/example/941/music-and-sound-example
Example Code:
System | On start of layout | [Audio] | Play name of your sound looping (tag "sound id tag")
-----------------------------------------------------------------
Mouse | On left button click on Sprite | [Audio] | Set "sound id tag" to -5 db
-----------------------------------------------------------------