While the Audio Mixer can only affect segments in their entirety, you can divide a Segment into multiple, separate segments (that is, sections) using the Add Edit function. Then, each Segment's Level or Pan can be adjusted differently from the other(s). To smooth the transition of the Level or Pan from one Segment to another, you can add a dissolve. The longer the dissolve is, the longer (and more subtle) the level or pan change will be.
Open the Audio Mixer Tool: Tools Menu | Audio Mixer.
Set the Audio Mixer Mode to Clip.
Enable the track(s) you want to affect.
Place the blue Position Indicator at the spot you want to divide a segment(s).
Press the Add Edit function. The Add Edit function's icon looks like the letter H turned on its side. By default, you'll find the Add Edit function in these locations:
i. Under the Record Monitor, in the top row of buttons, on the right-hand side.
ii. In the Fast Menu Tool Palette...