Click here to hear the Gibbons sing
For the first time ever, their songs have been recorded in Vietnam where they are almost extinct.
The recordings are proof that at least eight of the rare apes still survive in the area. The monkeys have been disappearing because of hunting and loss of habitat.
And if something's not done urgently, we may never hear their songs again.
This is according to researchers from conservation group Fauna and Flora International (FFI) who took the recordings on the Vietnam-Chinese border.
The endangered Tonkin Snub-nosed monkey, is found in just one part of the country.
Although schemes to help villagers protect the monkeys have been successful, the smaller apes have been ignored by the rest of the world.
People have been more worried about protecting other ape species like orang-utans. But they say it's now time to help before the recordings are all we have left of the creatures.