If you are not going to use DoubleCommand you should find out how to uninstall it. Dangling kernel extensions can cause problems down the road.
Look at System Preferences -> Keyboard -> Keyboard tab -> Use all F1, F2, etc. keys as standard function keys.
Is this selected? If so, unslect it, or hold the 'fn' key when pressing the mute, up/down volume keys.
With respect to keyboard remapping, I have found KeyRemap4MacBook to be most useful, and for creating Hotkeys I like to use BetterTouchTool (mostly because it makes my trackpad so much better, and hotkey creation is just a bonus).
But, again, if you are not going to be using DoubleCommand, uninstall it. NOTE: I do not have anything against DoubleCommand. I used it for years, until I found KeyRemap4MacBook. Mostly I make the "Right-Option" key another 'fn' key which makes it easier for me to use the arrow keys as page up/down keys.