How to control global Web Audio volume without specific AudioContext reference?

Recently my boss ask me to fix bugs in our old website projects. Problem happened when I tried to silence website media when the website is hidden.
The old project used Howler.js as its audio player. This lib seem used the Web Audio APIs to handle music sources, so that there is no audio element can be handled. What’s worse is that some old projects’ source code is lost, so I cannot use Howler reference to control the global volume either.

So I want to find out if there is a way to control global Web Audio volume without specific AudioContext reference? Or at least find the audio context created inside the website?