I usually solve this by using the Windows 10 volume mixer to increase the web browser's volume to a good level, and then turn NVDA down with the synth settings ring, or use the volume mixer to find a comfortable maximum for NVDA.
After I establish some normalized situation there, I then use NVDA's synth ring to change the volume as needed when I am playing or not playing other audio sources such as the browser, or Audacity, or whatever.
To get to the volume mixer, press Windows+B to get to your notification area. Then move down until you hear the volume object (it might say something like headphone/speaker volume 90% or similar).
Use your applications key, or press shift+F10 to open this menu, and move down to "volume mixer". Press enter on that, and you can then use tab to control the volume of the programs you have open.
Use alt+F4 to close the volume mixer. Or I suppose you could just alt+tab away from it and leave it open for later use.