You can try downloading IntelliType from Microsoft's website at http://download.microsoft.com/download/F/9/E/F9ED1BA8-4233-43C4-A915-8107CF84869A/ITP5_2Eng.exe.
This will take over the shortcut keys and hopefully sort out your volume problems.
I've been running SP2 on my D600 for a while now, and the volume buttons have been fine all the time.
Failing that, check that you have the latest audio drivers installed. If the volume doesn't change then there may be a communication problem between your machine and the soundcard. Does the sound work at the moment? Can you adjust the volume normally, by clicking the volume button in the system tray?