Flash Animation 15: Starting and Stopping Sound Effects

Because the bouncing snowball comes into the frame on an arc of descent, there won't be a sound at first--so I need to set the sound to stop on my first frame, so it won't start until I tell it to. To do that I've clicked on my first frame, and then in the Properties bar, selected Stop from the Sync dropdown. This sets the associated sound to stop right then, rather than play automatically. (The stop is signified by a little blue square atop the keyframe marker.)