Hello. Remember this thing? I sort of do, and here is an episode. Indonesian conservationist Sasha tells me about gibbons. Sasha has a longer name but this audio is repurposed from something else and we didn’t involve her employer’s Communications team so we’re staying on first name terms to keep it simple).
So here, with reasonable speed, you can learn about the sound of gibbons. There is much more to a say about that subject, books, papers, even whole websites, so this is just a taster.