During heavier combat scenarios with full MPAs (Most commonly during Cradle of Darkness, Dead Sea, and Arks City UQs) it's possible for sound effects to start playing even if all audio options are set to 0%. It's not even like it's a faint whisper, it's as if the audio is playing at full volume. I've also noticed that during these times the music will also cut out if volume is up. If I had to guess at a solution this'll likely require developer intervention to fix and probably has something to do with the available number of sound channels. Increasing available channels and setting music to higher priority than sound effects should go a long way to fixing this.
Video Demonstration: https://youtu.be/p9cLBHqpRnY