Audio Ducking

Nope. AE is not an editing app, it has very limited audio tools, editing any project with more than a half dozen cuts in it can get incredibly unwieldy. If you need to process audio for an AE comp that has to be a single comp either do it in Premiere Pro or Audition (Or Logic, or Cubase, or a dozen other apps) and import the fixed audio. 

I'm working on a 3-minute historical timeline for a documentary right now. I added markers in Audition to the scratch audio track, imported it into AE, cut up the audio track into sections that are no longer than a sentence or phrase based on the markers, then made a separate comp for each section.

The artwork is from an 80 layer Illustrator file imported as a comp. All of the elements are resting in their hero position.

I select the layers needed for each section in the master AI comp, copy and paste them in the appropriate section comp, set keyframes based on markers for each layer where they end, then apply the animation for each section. 

When a comp is complete it will be rendered with just the audio in the comp as a placeholder and the final 3-minute animation will be edited in Premiere Pro. There I will add multiple tracks. One for room tone, one for final narration, three or four sound effects tracks, and at least one music track. The final mix, EQ, and final stereo and Dolby adjustments will be done in Audition. I couldn't even come close to doing it all in AE and have the producer accept the result and the whole process is going to take less time than trying to cut the whole thing in one comp.