“Why are the gibbons making those noises?”
To “talk” to each other, gibbons sing back and forth, and this is called DUETTING. The vocalizations can be heard over a mile away!
Gibbons form a long-time bond as a mated pair, so when they start singing, mimicking this could be considered an interruption. If you want to practice sounding like a gibbon, you might want to wait until later!
PS: What is the difference between a monkey & an ape? Monkeys have TAILS! So our gibbons are actually APES!