There is no convention or consensus on any of these questions.
In my own Theme, Oenology, I use the standard template tag the_content() to get the content for almost every Post Format type. The exception is Images and Galleries, where I pull the image out separately, and use the_excerpt() as the image caption/gallery description.
A) They are intended to display whatever content is added to the Post.
B) Use the_content(), just as you would with a “standard” Post
C) Display it, either as per normal, or using some creativity for certain post format types. If you develop a Theme that makes certain assumptions about what content applies to specific Post Format types, then provide documentation within the Theme to explain those assumptions.
D) You can’t know. I just leave the raw link to the audio, and leave it up to the end user to choose an Audio Player Plugin.