Musical Dice Pro
In 1787 Mozart invented his musical dice game. Ever since musicians have been turning to musical dice for inspiration, now Sibelius can have this function!
This plug-in emulates musical dice. Click to roll and you get a random pitch, a random rhythm, or both. A fun little way to randomly compose music.
Not only is it the only built in tool for specifically writing aleatoric music in Sibelius, but it has a number of great features. In the Pro version of this plugin you can define your die set, choosing from specific scales or making a custom pitch set. Perhaps, the most powerful capability of the pro version is the ability to export your rolls as music up to 100 beats at a time (far more than you need to roll your melody).
If you still use real dice, or have never tried writing chance music, why not try out these software dice. If you use Sibelius to notate your random music, you're are missing out until you add this to your arsenal.