After Effects has never made audio one of its strong points. If you need to seriously rework a soundtrack, do it in a dedicated audio program. But if you just need to edit, mix, and do some basic improvement or “sweetening” to your soundtrack, After Effects already has the tools you need.
Audio is also an important guide for making animation decisions, plus it can drive some effects. But first, we need to get a handle on how sound itself works – after that, everything else makes a lot more sense.