How can I add a JavaScript keyboard shortcut to an existing JavaScript Function?

An event handler for the document's keyup event seems like an appropriate solution.

Note: KeyboardEvent.keyCode was deprecated in favor of key.

// define a handler
function doc_keyUp(e) {

    // this would test for whichever key is 40 (down arrow) and the ctrl key at the same time
    if (e.ctrlKey && e.key === 'ArrowDown') {
        // call your function to do the thing
        pauseSound();
    }
}
// register the handler 
document.addEventListener('keyup', doc_keyUp, false);

//For single key: Short cut key for 'Z'
document.onkeypress = function (e) {
    var evt = window.event || e;
    switch (evt.keyCode) {
        case 90:  
            // Call your method Here
            break;
    }
}

//For combine keys like Alt+P
document.onkeyup = function (e) {
    var evt = window.event || e;   
        if (evt.keyCode == 80 && evt.altKey) {
            // Call Your method here   
        }
    }
}
    //ensure if short cut keys are case sensitive.
    //    If its not case sensitive then
    //check with the evt.keyCode values for both upper case and lower case. ......

If you are only searching for trigger an event after pressing a key, try this:

In this example press 'ALT + a'

document.onkeyup=function(e){
  var e = e || window.event; // for IE to cover IEs window event-object
  if(e.altKey && e.which == 65) {
    alert('Keyboard shortcut working!');
    return false;
  }
}

Here is a fiddle: https://jsfiddle.net/dmtf6n27/38/

Please also note there is a difference for the keycode numbers, whether you are using onkeypress or onkeyup. More info here: w3 schools KeyboardEvent keyCode Property