Why is the volume control grayed out?

Hi,

I am attempting to record from a Sony ECM-MS957 mike that is connected to my iBook with a Griffin iMic onto GarageBand3.

I am following the "Learn to Podcast" tutorials from Apple. With the Sony mic connected the volume control in the track info window is grayed out. If I un-plug the Sony mic and use the built in mic the volume control becomes active.

I have the iMic selected in the System Preferences and the volume indicator responds to the mic. The iMic is also selected in the GarageBand preferences.

Can someone explain to me what I am doing wrong here.

Thanks