Is it permissible for the heirs to take a sum of money that has multiplied due to interest in the bank, or should they only take the original amount, and is there Zakat due on this money from the deceased, and how is it calculated?

The principal amount deposited in the bank belongs to the heirs and is to be distributed among them according to Islamic law. As for the usurious interest, scholars have differed regarding its lawfulness for the heirs. The more preponderant opinion is that it is not lawful for them, and it must be disposed of by spending it on the welfare of Muslims or on the poor. As for zakat, if the heirs predominantly believe that their father did not pay it on the amount, they must pay it for all past years before distribution. If they are unable to ascertain this with certainty, they should act upon their predominant belief. If they do not know and have no predominant belief, the default assumption is that a Muslim fulfills what Allah has obligated upon him, so they are not required to pay it.