Is expiation (kaffarah) obligatory for someone who swore not to go to a party with music, then intended to go, but did not actually go?

What you mentioned is called the intention to do the opposite of what was sworn upon, and it does not necessitate expiation according to the majority of scholars. Therefore, you do not owe any expiation merely by intending to attend the wedding, as long as you do not actually attend it. As for attending weddings that include forbidden acts, it is impermissible, even if it is to please family.