I’m not sure if it’s because my MacBook is older and I never update it, but I had to download a separate application that lets me change the volume of specific windows. The one I use is called BackgroundMusic, it’s free and I find it easy to navigate but there’s all sorts of other options I found on Google!
If you’re on Mac you might have to download a separate program to do this. Not sure what you’re working with but I have a 2013 MacBook Air that I always procrastinate on updating and I needed to download something separate. I use something called BackgroundMusic, it’s free and lets me change the volume of any of my apps, just like the Volume Mixer on Windows does