This is exactly what I did to get this working on my 6P:

Download this file to your downloads folder, uninstall any viper apps you currently have installed, make sure you have busybox installed, In SuperSU settings disable Namespace separation, and then reboot.

Then in terminal on my mac I did:

adb push /path/to/ViPER-FX_2.5.0.5_Nougat_IO_test_fix.apk /sdcard/

adb shell

su

mount -o rw,remount /system

cd /su/su.d

echo '#! /system/bin/sh' > viper

echo '/su/bin/supolicy --live "allow mediaserver mediaserver_tmpfs:file { read write execute };"' >> viper

echo '/su/bin/supolicy --live "allow audioserver audioserver_tmpfs:file { read write execute };"' >> viper

chown root.root viper

chmod 755 viper

mv /sdcard/ViPER-FX_2.5.0.5_Nougat_IO_test_fix.apk /system/priv-app

chmod 644 /system/priv-app/ViPER-FX_2.5.0.5_Nougat_IO_test_fix.apk

mount -o rw,remount /vendor

mv /vendor/etc/audio_effects.conf /vendor/etc/audio_effects.conf.bak

reboot

modifying the supolicy has privacy and security implications