From the given statement "No monkey can sing, but some monkeys can talk," we can derive the following conclusions:

All monkeys can dance: We cannot conclude this because the statement doesn't provide any information about monkeys' ability to dance. It only mentions singing and talking.

Some monkeys may sing: This conclusion contradicts the initial statement, which says, "No monkey can sing." Therefore, this conclusion is not valid.

All monkeys can sing: This conclusion contradicts the initial statement, which clearly states, "No monkey can sing." Therefore, this conclusion is not valid.

All monkeys can't talk: This conclusion is valid based on the initial statement, which says, "No monkey can sing, but some monkeys can talk." Since it's established that not all monkeys can sing, it's valid to conclude that not all monkeys can talk.

So, the valid conclusion is: All monkeys can't talk.