Wait for the entire screen to load up with all four horses and a fence in front of them.
Then click on each horse. Make sure your sound is on.
Re-click on any horse to make it turn off or turn it back on again.
Somebody did some real wizardry of programming to coordinate this!
Try clicking on the horses from left to right then right to left
then just one or two at a time...
It's fun and a good stress relief. Have fun!