interited doesn't work as it should indeed. What you can do is call the method on the superclass through apply:
dijit.form.HorizontalSlider._handleOnChange.apply(this, arguments)
This assumes the function is available and visible in the HorizontalSlider prototype.
Otherwise you could monkey patch the function in your constructor (post create) by keeping the original function in the closure of the new one:
function postCreate() {
var original = this._handleOnChange;
this._handleOnChange = dojo.hitch(this, function() {
this.MyCustomChangeHandler(arguments); //your code
return original.apply(this, arguments);
});
}