Audio lines allow you to pass audio from one element to the next. The best way to think about audio lines is as a series of audio snakes -- each can be patched from one element differently than the next.
Audio Line Mixer
By clicking on the circular button in the middle of the audio line, you will see the audio line's mixer.
The audio line mixer is how you route the audio coming out of the source element to the destination element. Each element can hava multiple audio inputs and outputs, so you can change where you want the source's outputs to go to on the destination's input.