This line of poetry comes from Li Daoyuan's "Three Gorges," and it was indeed part of the middle school second year Chinese language textbook!
Tang Xiaoxin had never heard what "gibbon song" sounded like; back then, while memorizing this line of poetry, she could only refer to the noisy and squawking calls of the macaques at the zoo, always quite puzzled.
Such a noisy and disturbing sound should be quite headache-inducing and annoying to listen to, so why would the ancients feel so sad upon hearing it that they teared up and dampened their clothes?
Thinking it over, she reckoned that perhaps hearing their calls reminded them of the monkeys back in their hometown mountains calling in the same way, stirring up feelings of homesickness, which in turn led to the tears.
Now, listening to the gibbon's sonorous and melodious song, she finally had some understanding of how the ancients felt.
It's hard to describe their song in words!
