Just click the green flag. Remember that Scratch's sound can get quite distorted and put off the timing of the project, so if you want decent quality you probably shouldn't be doing anything else on your computer. Alternatively, if you're too busy, you can visit the link below and listen while you do something else:
https://soundcloud.com/evangambit/a-dream Written by me using Musescore (free, cross-platform musical composition software).
