Click when you hear the sound. Use the slider to adjust the volume, and make sure your speakers are on. Linux users: please read the note below
There seems to be a noticeable delay between the program playing the sound, and the actual sound output in Linux; i.e. your measured reaction times will be significantly slower than actual (visit badgerbadgerbadger.com to see what I mean: after the first loop, the mushroom scene appears before the sound is played). I will attempt to normalize this later on.