At the top, you will see a blue button to play the mp3.
Just below that, you will see the score, if you have the Scorch plugin. Otherwise, a blank where the score ought to be. Here you can view the score and listen to the Scorch playback (GM), not a good sound, usually.
Below this, under the heading of details, you will see a choice to download the mp3 file, which is the best choice if you want to hear, and don't have broadband. The mp3 is the GPO Organ.