Okay, I seem to have identified the problem in a reproducible way on my end, was hoping for others to try experimenting and see if it's the same for them.
On game startup, all actual volume output is low, maybe 20%-50% of the volume you have set on your sliders. The slider settings don't move to reflect this. If you click on any slider controls for volume, that particular volume is corrected to full expected volume. (eg: Click on music slider and the music volume snaps back to the correct gain.)
Clicking the master volume slider corrcets them all at once, and if you have the master volume slider all the way up like I do you can just hit the little arrow to increase it (no change since already maxed) and that fixes your volume levels.
Alt+tabbing out and back to the game re-quiets the levels, requiring another trip to options and a click on the "master volume increase" arrow.
As a person who is constantly alt-tabbing during my gaming experience, this is getting old fast.