MacOS System Audio in the Browser
TL;DR traditionally gaining access to system audio in MacOS is complicated. In fact the most popular workaround (Soundflower) is outdated and I outline a new solution.
The Solution
Let’s cut to the chase… We’ll talk about old solutions and browser access later.
eqMac is a well-built, open source equalizer for MacOS. (currently Beta v0.3.6)

It automatically configures an audio device which routes all the sound on your Mac through it.
This is what you previously needed to manually configure with Soundflower.
Once eqMac is installed and you’ve chosen “eqMac” as your machine’s audio device you will then have access to system sound for most usage.
Including a very useful equalizer. Done.
System Sound as a MediaStream
Many resources exist for understanding AudioContext, AnalyserNode and MediaStreams. However, gaining access to system sound from these devices seems to be purposefully difficult and impossible(?) without customizing your own system’s audio devices.

If there’s a better way please let me know on Twitter @BenRacicot.