The BGM on pia.com is generally produced or uploaded by users themselves. There are many light music on many music software that can be used as BGM for pia. It depends on what type of pia you are writing.

Here is a small suggestion:

If you like PIA in an ancient style, you can use the original OST of Fairy Sword. Black Wind, Thousand Powers of Divine Power, and Millennium Elegance are all very good.

BGM like Huanqin can be used for relaxed articles. There is also one called "Our Gone Home", the light music in it is also very good.

If it is a modern book, you can use popular music accompaniment.

In fact, as long as the music and the atmosphere and situation of the script match harmoniously, it will be fine.