update setMute and getMute for createjs.Sound

createjs.Sound.setMute
http://www.createjs.com/Docs/SoundJS/files/.._src_soundjs_Sound.js.html#l1151

createjs.Sound.getMute()
http://www.createjs.com/Docs/SoundJS/files/.._src_soundjs_Sound.js.html#l1180
Merge pull request #817 from goldenratio/patch-1

update setMute and getMute for createjs.Sound