The sound of a big string is like the sound of heavy rain from a strong wind.
Little fairy's voice is like the voice of two people chatting in a low voice.
The sound of big strings and the sound of small strings are played alternately, just like the sound of big pearls and small pearls falling on a jade plate.
The first sentence uses metaphor to compare the pipa sound to "urgent rain", which highlights the speed and momentum of music.
The second sentence uses personification to compare the sound of pipa to two people whispering, which highlights the gentleness of music.
The third and fourth sentences combine the big string and the small string by metaphor, which is likened to the sound of pearls falling and highlights the crisp music when the ensemble is played.