My female has 2 OOP joeys that have been OOP for about 4 or 5 days.
Have you seen the joeys actually detached from the nipple? or is mom just taking them OOP for grooming then stuffing them back in.
Generally the 'singing' which is really more of an uncomfortable noise begins several days before the joeys actually detach so mom can actually leave the pouch to eat and get a little exercise with out them. Joeys are not considered OOP until they have actually detached even though mom may take them out of her pouch for periods of time for a week or so before they actually detach.