There are no built in shortcuts to control the volume. You can create a shortcut to open the volume control in the taskbar but it won't work when a full screen app is activated (
Volume Shortcut - Create). You can write a script using
AutoHotkey, you should be able to find a few scripts via Google.
No matter which way you create a keyboard shortcut you have to be careful what key combination you use because if certain apps have focus and the app already uses that shortcut for some function then it won't work.
Anyway how many programs is this a issue in? Most apps will have there own shortcuts and if it's not a fullscreen app it only takes 2sec to click the taskbar icon and lower it if you do what seavixen posted.