There are several hints here on remapping the function keys on a standard PC keyboard when used with a Mac (in my case, a Mac mini). The standard key map does use F8 to F12, but lacking are volume controls, shut down switch, and others. Several people suggested using ControllerMate or Keyboard Maestro to handle this problem. Here's another solution...
Somewhere, I found a hint suggesting to use Griffin's Proxi to solve this problem. After installing Proxi, create a new trigger by using "insert Trigger" from the little gear icon, then select Hotkey Monitor. You can then choose the desired key. Then select a new task (Insert Task, Key Press). I choose to set up volume control on F4/F5, and mute on F3. This works like a charm.
Mac OS X Hints