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There's a possible way to get this done using a bit of Javascript, a variable in Storyline, and a trigger to pause the timeline. If you're not comfortable with Javascript, it could be daunting to implement.

The script is described in this stackoverflow post:

http://stackoverflow.com/questions/1060008/is-there-a-way-to-detect-if-a-browser-window-is-not-currently-active

You'd want to trigger something that incremented a number value in a storyline variable then use a "when variable changes, pause timeline" trigger.  This would let the browser window detect when focus is lost or the window goes into the background.  In a Javascript trigger that fires when the slide loads that you want to have the video pause:

var hidden = "hidden";

var player=GetPlayer();

// Standards:
if (hidden in document)
document.addEventListener("visibilitychange", onchange);
else if ((hidden = "mozHidden") in document)
document.addEventListener("mozvisibilitychange", onchange);
else if ((hidden = "webkitHidden") in document)
document.addEventListener("webkitvisibilitychange", onchange);
else if ((hidden = "msHidden") in document)
document.addEventListener("msvisibilitychange", onchange);
// IE 9 and lower:
else if ("onfocusin" in document)
document.onfocusin = document.onfocusout = onchange;
// All others:
else
window.onpageshow = window.onpagehide
= window.onfocus = window.onblur = onchange;

function onchange (evt) {
var v = "visible", h = "hidden",
evtMap = {
focus:v, focusin:v, pageshow:v, blur:h, focusout:h, pagehide:h
};

evt = evt || window.event;
if (evt.type in evtMap){

player.SetVar("yourNumberVariable",Number(player.GetVar("yourNumberVariable))+1);
}

}

// set the initial state (but only if browser supports the Page Visibility API)
if( document[hidden] !== undefined )
onchange({type: document[hidden] ? "blur" : "focus"});
}