Here’s a guide to successfully install Viper4Android and make it work system-wide on Nexus 6P running Android Nougat (7.0 / 7.1).
Starting from the very beginning:
Flash the Factory Image from Google. The phone will boot after flash.
Turn off the phone and go into bootloader mode. Connect it to the computer.
Execute “fastboot format userdata” on the computer to disable encryption.
Flash TWRP using “fastboot flash recovery twrp.img”.
Flash a custom ROM and/or custom kernel at this point if you want to. I’m using the stock ROM with Flash Kernel.
Boot into the system and go through the setup.
Go back to TWRP and flash SuperSU.
Flash the System-wide Audio Fix to make Viper apply to all apps.
Open the Viper4Android app. Install the driver. Reboot.
Test to see if Viper works now.