That's a safety design feature of the Volt software. When you are driving, you are only given a short list of options from your library - they don't want you scrolling through long lists when your attention should be on operating the vehicle. I guess VW are not so safety conscious!
The same is true of the built in navigation option - you cannot type in an address when the car is in 'drive'. In that case you can put in an address using the voice system when the car is in 'drive'. Perhaps there is a similar option for using voice commands to choose something specific on your Apple library list.