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Score Viewer - Playback
Clicking the play button in the viewer starts playback of your score. A playback marker is drawn on the score indicating progress. Page turns are handled automatically, so that the music currently being heard is always visible on screen.
Sibelius Cloud uses the same Sibelius 7 Sounds library that can be found in Sibelius itself.
The length of playback is linked to the number of visible pages - so, if your score is four pages long, but you've specified that you'd only like to display the first page of that score, then only the first page of the score will play back.
Playback is not available for PDF-based scores.