It's not clear what you are wanting to achieve, but fade ins are at the beginning of an audio event and fade outs are at the end. However, you can create your own audio levels by using an Audio Envelope. There are several ways of in initiating an Audio Envelope, but a straightforward way is to hit keyboard V with the curser sitting over the audio timeline. A blue envelope line will appear along the middle of each audio track. Double-clicking on a particular point on the envelope line creates a node (keyframe) and using nodes you can adjust the volume by moving the node up or down. An example:

Right-clicking on a node gives many other options.