One of the main tricks used in audio editing is Cross-fades. Used to Smooth the boundaries between regions where clicks or pops occur, but also used for plenty of creative ways.
Creating a Cross-fade with SMART TOOL
1. Press both F6 & F7 to activate smart tool or have or have the select the all three of the editing tools. Move the mouse close to a region boundary, at the bottom of the track. Cursor should change into a cross fade symbol. Click and Drag the cursor in either direction, and cross fade should appear, centered at the boundary.
2. Once you have the ideal fade, release the mouse, and the displayed cross fade will finalize.
Standard Crossfade Creation
1. With the Selector Tool selected, select the start and end point you want for the crossfade.
2. Click Edit on the Top Bar, chose Fades, Create. (Can also use Command + F). Now the Crossfade dialog box window should open. Select which preset or type you would like to use for the crossfade.
3. Once you have your perimeters all set, click OK.