To play your purchased music, tap the play icon in the top toolbar of the Viewer window. The audio data for the piece will load, and the menu will switch into playback mode.
Once loading is complete the sheet music will play from the start of the current page to the end of the song while highlighting the notes in time with the audio.
Please note: The playback system advances pages for you in order to keep the playing music in view. You will be able to manually turn pages only before pressing play or after the music is stopped.