Step by step:
1. Switch to the Timeline view. The 2 little icons mean "Clips" view or "Timeline" view. Click on the little clock. (left side, below preview window)
2. Select a clip and do Advanced > Extract Audio. Keyboard shortcut is Command J. Now when you edit, you have to select the video clip (blue) or the audio clip (purple) before making a change. The menu items will change depending on which type of clip is selected. Or select nothing to preview your movie. Here is a video clip with its audio in a separate track below:
3. View > Show Clip Volume Levels. You will see a purple line through your audio clips:
4. You can drag that purple line up and down to raise and lower the volume of that clip. With experimentation, you will learn that the volume can be changed instantly, or you can stretch a fade-in or fade-out over time.
5. It helps to magnify the size of the timeline. Move that slider to the right to magnify and give you more control. You can also type a % into the box to set a specific volume level.
Enjoy.