Wire up Galleria to detect YouTube and Vimeo videos playing so it pauses the slideshow while videos are playing.

galleria-video-pause.js

var autoplayLength = 8000;
var gallery = false;
var ytplayer = false;

$(function() {
  if (window.addEventListener) {
		window.addEventListener('message', messageReceived, false);
	} else {
		window.attachEvent('onmessage', messageReceived, false);
	}

	$('#slides').each(function() {
		Galleria.ready(function() {
			gallery = this;
			// see if we need any youtube stuff
			var count = this.getDataLength();
			for (var x=0; x < count; x++) {
				var data = this.getData(x);
				if (data.iframe && data.iframe.indexOf('//www.youtube.com/') > -1) {
					// load up youtube js
					var tag = document.createElement('script');
					tag.src = "//www.youtube.com/iframe_api";
					$('body').append(tag);
					break;
				}
			}
		});
		Galleria.on('image', function(e) {

			if (e.imageTarget.src.indexOf('//player.vimeo.com/') > -1) {
				// vimeo video
				var id = e.imageTarget.id;
				var url = e.imageTarget.src.split('?')[0];
				var vimeoWin = $('#' + id)[0].contentWindow;

				vimeoWin.postMessage(JSON.stringify({method:'addEventListener', value:'pause'}), url);
				vimeoWin.postMessage(JSON.stringify({method:'addEventListener', value:'finish'}), url);
				vimeoWin.postMessage(JSON.stringify({method:'addEventListener', value:'playProgress'}), url);
			} else if (e.imageTarget.src.indexOf('//www.youtube.com/') > -1) {
				// youtube video
				ytplayer = new YT.Player(e.imageTarget.id, {
					events: {
						'onStateChange': function(e) {
							switch(e.data) {
								case YT.PlayerState.BUFFERING:
								case YT.PlayerState.PLAYING:
									gallery.pause();
									break;
								case YT.PlayerState.ENDED:
									gallery.next();
									break;
								case YT.PlayerState.PAUSED:
									gallery.play(autoplayLength);
							}
						}
					}
				});
			}

		});
		Galleria.run('#homeSlides', {
			autoplay: autoplayLength,
			responsive: true,
			thumbnails: 'empty',
			showCounter: false,
			slideshow: true,
			vimeo: { api: 1 },
			youtube: { enablejsapi: 1 }
		});
	});
})

function messageReceived(e) {

  var data = JSON.parse(e.data)
	switch(data.event) {
		case 'playProgress':
			// need to pause player
			gallery.pause();
			break;

		case 'finish':
			// go to next slide 
			gallery.next();
			break;

		case 'pause':
			// start the slideshow again
			gallery.play(autoplayLength);
			break;
	}
}