Complex sentence: When content, it emits a sound similar to a soft sweet bird-like twill, and when several tarsiers come together, they have a chirping, locust-like sound.
Split into simpler sentences: When content, it emits a sound similar to a soft sweet bird-like twill and thirdly a locust like chirp between close tarsiers. And when several tarsiers come together, they have a chirping, locust-like sound.