Queen steals sound, but boy wins in the end

Looking for the title of a book from the 1970s, I think, in which a
wizard sends a boy on a dangerous but vital mission. Among the boys
adventures is one in which an evil queen has stolen sound. The boy
defeats the queen by stuffing words into a cannon, with which he
destroys her castle.

When the boy returns to the wizard victorious, the wizard tells him
his mission had been impossible, but since he was too young and
idealistic to realize that, he was the only person who had a chance to
win anyway.

Does this ring a bell with anyone?

Thank you in advance for any pointers or tips.

Karen