For some reason Xiaomi uses different mixer settings between global and China ROMs. The ones in China ROM give you substantially louder volume output.
Those settings are stored in 3 files:
/system/etc/mixer_paths.xml, /system/etc/mixer_paths_i2s.xml, and /system/etc/mixer_paths_tasha.xml (I think that only the last one is actually used by the system, but I didn't check it). To fix the audio volume problem you have to extract those files from the China ROM and overwrite the ones on your /system/ partition. Then you just need to reboot and done.
Here are those files from China stable ROM 126.96.36.199: https://drive.google.com/drive/folde...lU?usp=sharing