Function to trigger animation with mouse wheel events
This:

var delta;
if (event.wheelDelta) {
    delta = event.wheelDelta;
} else {
    delta = -1* event.deltaY;
}


could be simplified to:

var delta = event.wheelDelta ? event.wheelDelta : -event.deltaY;


or even more succinctly:

var delta = event.wheelDelta || - event.deltaY;


For the if/else:

if (slidenum &gt; 0) {
    slide1.play()
} else if (slidenum &lt; 1) {
    slide1.reverse()
}


I personally find your end points take me a while to understand because of the way they overlap. I would find it easier like this:

if (slidenum &gt;= 1) {
    slide1.play()
} else if (slidenum &lt; 1) {
    slide1.reverse()
}


Or without the else which is redundant in this case.

if (slidenum &gt; 0) {
    slide1.play()
} else {
    slide1.reverse()
}