Please define "BT is OK" .... Did you test with headphones, using a music player and/or an online radio app?
I have tested it also with XT 1097, last build, pico gapps, magisk 14.0, but no viper4android...
XT 1097 - LOS 15 (last build) + OPEN GAPPs Nano + Magisk 14.3 + Viper4Android ( https://forum.xda-developers.com/moto-x-2014/themes-apps/mod-viper-4-android-oreo-tested-t3700287 ).
BT ok and sounds great!
XT 1085 not working
XT1097 working fine.