So, I've been having this issue for a long time now. Just have always kind of put up with it. It happens everywhere. Doesn't seem to be excluded to one area. Sometimes the audio gets louder/higher and sometimes the audio gets quieter/lower. I've tried searching google but not really finding much. Maybe I'm not using the right search terms. Surely I can't be the only one having these issues. So I thought i'd try posting here and see if anyone knew of any fixes.
Here is a link to the video I captured off of my XBOX One X.
I've trimmed the video clip to just focus on the audio issue. You'll notice the audio immediately gets noticeably louder just after beating the zombies.