If you are referring to the "Google Play Music" app, it has a Dolby Audio equalizer in it's settings.
1. Turn on the Dolby Audio equalizer
2. Tap the "Music" tab.
3. Tap "RESET" .
4. At the bottom of the equalizer is a "Volume Leveler", turn it on to increase overall volume.
See if that helps.
Here are the four suggested settings, if you do not see Reset you can disregard searching for it, it appears only if you have enabled the Intelligent Equalizer. The Volume Leveler is at the bottom.
On your phone browse to the App Info window for the Google Play Music app.
The path is Settings > Apps & Notifications > App info > All apps > Google Play Music. (The browsing path may be different on your phone)
Is this what your Google Play Music App Info window and Version looks like?
Our versions of the Google Play Music app are the same. It is our Equalizers that are different.
My phone is a Moto G6. When I use the Google Play Music app it accesses and uses the Equalizer and Volume Leveler within the pre-installed Dolby Audio app.
Your phone is a Note 9. When you use the Google Play Music app it accesses and uses the Equalizer within the Dolby Atmos app, which is a different app.