Yu'er plays the pipa~ Prison Snow Jingchuan's answer is right~ and the instrument played by Chen Jingchou is a flute (it sounds like it) (it looks like a Jiangnan bamboo flute). Scar24 is actually background music that only appears when the plot is quite tragic. Then Scar10 is on the fairy island, before finding Immortal Gu Yue to treat Yu'er, Yu'er woke up in the middle of the night while Xiaoxue and A Qiu were sleeping, secretly crying, and A Qiu used the background music when he woke up to comfort Yu'er. This one is played by a flute. If it's Yu'er's pipa, Scar15 was played by Yu'er on Yu'er's pipa in Kuoba tribe's tent~Scar18 and Scar19 are also the same tune played on pipa, although in different keys~