For me it's easier to adjust the windows volume via shortkeys or the taskbar icon thingy (just click the icon and then use the scroll wheel for adjustments just to be a little fancy

).
So my speakers are set to a reasonably loud volume and then the windows sound turned down below 100% so there's still a little room to go louder without changing the speakers.
In games probably default or adjusted once in the options menu. I like games that put the volume sliders in an accessible place but not many games do that. In Trackmania 2 you can push the cursor against the top of the screen at any time and get Music / SFX sliders and other miscellanous buttons no matter where you are in game (or in menu). That's nifty. In SolForge the sliders are in the corner of the main menu (and in a CCG you're either ingame or in the main menu pretty much). But other than that I can't think of any game that does something like that which is a shame.
If the sound is less important on a game I might go into the windows volume mixer and turn it down there. Windows will always remember those settings too, so for example whenever I install e.g. Skype it gets instantly muted because I hate the sounds. xD
And it's easier than going into application-specific convoluted menues everytime I just want to mute something for a while.
~RedFlames