You can change volume setting on Stadia configuration (push Stadia button on controller e nove to headphone icons) and of course on game settings.
Personally the level is good, I connect simple phone mobile headset, the OnePlus Silver Bullet, and the level sound it's loud.
I was having an awful experience until I noticed my aux cord wasn't plugged in fully to the headset. Make sure you jam that thing in there because some aux cords have trouble being fully connected, especially if you're using a cord that didn't come with the headset.
Now if you're using wireless...well...I'm not sure about troubleshooting that :P.