I've looked through the forums and found this post on cross-fading images and video
Brian Riggs suggest putting the elements in DurationElements inside a Parallel elements.
How would this work for audio?
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.