Does the unit above you have carpeting? If not, does the building have rules about how much of the hardwood floor has to be covered with rugs? Between that and putting carpeting and fabric window treatments in your own home, there's not much else to be done in a place you don't own. That said, look for decorative acoustic panels you could hang on your walls. They're often used in commercial situations like restaurants, but they make them for residential use, too.