Assuming you are in Storyboard mode then You need to switch to Timeline mode - click the button indicated in the image below, then you can use the editting tools to split the song.
If you clcik the audio object youwiill get some 'handles appear as shown in the image below - this is a video object however it is the principle is the same for images and audio as well - the handles work as follows:
drag the handle at the end of the object, circled in the image, to the left to fade out
drag the handle at the start of the object - to the right to fade in
If you have not done so, read through the installed pdf manual available under Help