Deleting Audio in Slideshow CS3

Hi,

I've had a good search so I'm assuming its me doing something wrong.

I'm using Encore CS3 and have created a slideshow. However I want to remove one
of the audio tracks and insert another one.

I'd guess that I would highlight the track and press delete, or drag the audio
file onto the "trash" symbol.

However, both of these options don't work.

I've noticed this in other project and recreated the slide show previously.

Is this a know problem or am I doing something wrong?

Many Thanks for any pointers.

      Advice?