It works fine with my bluetooth headset. If you aren't using a headset, you may need to create a shortcut to do it easily.
1. Go into Settings, Key Settings, FN Key Settings
2. Select Show List
3. Select Menu, Add
4. Find an available key and go through the list of apps until you find Voice Command (a shortcut to finding it is to press the V key as many times as necessary to cycle thru the V applications. It should find Voice Note and Voice Command.
5. Then, the Fn+ whatever letter you selected for the shortcut should launch Voice Command.