Record audio with Web Audio Api

I have a web app that plays many mp3s in parallel implemented with Web Audio API. I want to make the user able to record everything that comes out of the Master Node of the node chain (the recording starts with a button click and ends the same way) and download it as wav (or any other format).

I don't want to record the microphone of the user's pc, I want to record what the app is playing at the moment.

Can anyone help me ?