Drum machine - Building Movies Interactively

I want to allow users to build their own movies but not sure where to start. Basically, the premise is that the user can build simple drum loops by dragging and dropping movie clips (that have drum sounds in them) to a point on the stage (which is a graphical representation of a bar of music). This I can do, but what I then need to happen is for the user to be able to play back the bar of music that they have just created … anyone got any ideas on how I would go about this?! Thanks,

t