The problem is that you are multiplying the HTMLElement it self and not its value. So add .value after radius, like so:
var spherevolume = (4/3) * Math.PI * Math.pow(radius.value, 3);
Unfortunately this will return a string and not a number, so to convert it to a number, you could either wrap it in a parseInt() or force it to turn into a number (i.e. subtracting it by 0).
var spherevolume = (4/3) * Math.PI * Math.pow(parseInt(radius.value), 3);
Or
var spherevolume = (4/3) * Math.PI * Math.pow(radius.value - 0, 3);
On a side-note, you should probably add the EventListener on the button and not on the span. I'm assuming you did not do that because the whole thing is in a form and thus it was redirecting the page. You can avoid that by adding a event.preventDefault();
var button = document.querySelector("#radius+button");
button.addEventListener("click", function(event){
var spherevolume = (4/3) * Math.PI * Math.pow(parseInt(radius.value), 3);
volsphere.textContent = spherevolume;
event.preventDefault();
});