You can use the postMessage API to communicate with the iframe. This would allow you to send a message to the iframe telling it to stop the audio.
A script in the parent window:
// Assuming the origin of your iframe is http://example.org
function hideTetrisIframe(){
var iframe = document.getElementById('tetris');
iframe.style.display = 'none';
iframe.contentWindow.postMessage('stop', 'http://example.org');
}
A script inside the iframe:
window.addEventListener('message', function(event) {
if (event.data === 'stop') {
// Stop audio that's playing
}
}, false);