Support for Audio is currently a work in progress on signageOS SMIL Player.
<audio src="music.mp3" />
Sound Volume Control
For video and audio objects, you can specify the "sound level" attribute to control their audio volume.
This feature is in development.
<video src="ad1.mp4" soundLevel="20%" />
Valid values for soundLevel is from 0% (mute) to 100% (max).