^ Or better -
[Moved from General Autism Discussion to Computers, Math, Science, and Technology]
You might be able to use a screen recorder to create a video of the Sibelius playback, but you'd probably need some careful tweaking to keep it in sync with the music playback on a YouTube clip.
Then again, maybe the screen recorder could record sound along with the video - in which case, if you're using the sounds provided with Sibelius, job done.
An easier alternative might be to get the score displayed as a few static pages, "turning" at the correct points in the music, leaving the viewer to follow the score.
Edit: I just found this:
http://www.wikihow.com/Record-Screen-in ... -Windows-7
There's probably something similar available if you're running Windows 8 - or maybe this will work there too.
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Giraffe: a ruminant with a view.