It's important to note that the Audio Mixer Tool has three different modes, each allowing different capabilities and functionality. The different modes are toggled using the Audio Mixer Mode Button (see letter F in the screenshot below).
Also see the previous Understanding the Audio Mixer Tool: Clip Mode recipe for additional information not presented here.
Tools Menu | Audio Mixer.
Windows | Workspaces | Audio Editing.
Consider mapping the Audio Mixer menu selection to a convenient location on your keyboard. For example, I've placed this on Shift + A. More information on mapping buttons and menu selections can be found in the Mapping buttons and menu selections buttons recipe in Chapter 2, Customizing Your Work Environment.
Click the Audio Mixer Mode button (letter F in the previous screenshot) so that it displays the text Auto.