Empress Mung is the main antagonist of the Letter People three-part episode "Star Trip." She rules a fictional planet named Snickers, and her villainy is kidnapping a Letter Person.
Initially, her hostage is Miss A, but then Mister R arrives, and they both make the same sound as Mister R's name, impressing Mung so much that she releases Miss A, but she keeps Mister R captive.
Much later, Miss O arrives, and so they make a new sound: the "or" sound, so Miss O gets to leave, but again, Mister R remains.
Finally, Empress Mung captures Misses E, I, and U, and they work with Mister R to respectively take turns making the same sound ("ur"). When Mung's minion reveals this, she decides to let them go.