I don't know if it is because of Chrome updates, but the previous answers did not work for me. I modified a bit of the code here and came up with this:
document.addEventListener("play", function(evt)
{
if(window.$_currentlyPlaying && window.$_currentlyPlaying != evt.target)
{
window.$_currentlyPlaying.pause();
}
window.$_currentlyPlaying = evt.target;
}, true);
I don't know why, but the widow.addEventListener was not working for me, but I liked the idea of having the currentPlaying variable stored in the window element instead of having to create it outside of the listener prior to using it.