This is a simple demo of canceling a "play sound until done" block by restarting the sound while it's playing. Press the green flag to start a sound with "play sound until done". Then, click the sprite. That restarts the sound with a new "play sound until done", stopping the running one, which will display a message as a result. Last, press the space key. It will restart the sound with "start sound", again stopping the running "play sound until done", and again displaying a message as a result. (Note: The "tindeck2" sound is saved an MP3 file. This doesn't affect the behavior described above, but does let scratch-js play the sound without any additional code to handle playing WAV files, which generally aren't supported by browsers otherwise.) Credits to cynicalCanasian and Morrowind for the song, "nerevar rising (with meows)", from the Prequel page right after Katia: Depart.
