Sound Fade in/Out Matinee

I am trying to work out how to fade a sound effect in or out using Matinee. It is for a rain sequence where the rain sound fades in and out to match the visuals.
I have been adding sounds from the browser and triggering them in the timeline with no problems and setting the intial volume when the sound file starts is works with a keyframe however I cant work out how to get the volume to fade in/out. If I try to add another keyframe along the sound bar in the timeline it just adds a new keyframe playing the sound from the beginning again. Any ideas?

Thanks in advance