Click the GF and then click Anywhere to start or restart the song. However, when run under Scratch 3.0 the sensing of the mouse down is blocked when a monitor is clicked. You cannot start the song in Scratch 3.0 by clicking one of the two monitors.
The music is the base part to boogie-woogie. It's in the public domain and this is something that I played so therefore there is no license issue.