How do I convert audio from the command line a la ImageMagick?

Yep, it's called SoX

SoX is a cross-platform (Windows, Linux, MacOS X, etc.) command line utility that can convert various formats of computer audio files in to other formats. It can also apply various effects to these sound files, and, as an added bonus, SoX can play and record audio files on most platforms.

Another Soft: moo0 ffmpeg converter - converts audio files using CMD (command line).

i.e.:

c:\folder\bin\ffmpeg -i input.mp4 output.mp3