• First, you have to follow the basic rules of V4A such as no other audio mods (AudioFX, MusicFX,...), uncheck direct control volume in Poweramp...
• Since Oreo, I only flash this Magisk module and I no longer have to follow the below steps.
1. THE PROBLEM
• You are using ViPER4Android app and the V4A module from the official repo of Magisk.
• Driver is installed, V4A is enabled but Processing is stuck at NO!
2. THE SOLUTION
• Move /magisk/v4a/system/etc/audio_effects.conf to /magisk/v4a/system/vendor/etc/audio_effects.conf (check the permissions).
Note : overwrite the existing file or add the extension .bak
Now start your music app and check the driver status screen of V4A. You should have this:
I found this solution in one of Magisk threads. Hope it will help other OP3T users.