I've looked through the forums and found this post on cross-fading images and video
http://forums.adobe.com/thread/670293?tstart=0
Brian Riggs suggest putting the elements in DurationElements inside a Parallel elements.
How would this work for audio?
ParallelElement
AudioElement(audio #1)
AudioElement(audio #2)
I add audio #2 10 seconds before the end of audio #1
I tried playing with the individual elements volumn but it seems to affect all the elements inside the ParrallelElement.
Couldn't find a definitive answer but I looks like I'm going to have to create a custom Element. Something like ParallelElement.