To access the Volume Mixer simply click on the volume icon in the taskbar and click on the Mixer link below your sound device. This will display volume controls for your open applications which you can modify as you like.
With these settings in mind you can now use different browsers for specific activities based on your sound preferences. You could open YouTube in Chrome with higher volume settings than the pages you are browsing in Firefox. That way if a website decides to automatically play sound it does not disturb as much. You can also use this options to completely mute certain applications so that you hear no sound from them at all.
Related Links
LifeHacker: Set Volume Levels for Individual Programs in Windows