Allow multiple sound sources to play at same time

I have a laptop running Ubuntu MATE 20.04, I have MIXXX installed an Firefox, if I am playing music on MIXXX while listening with my earpods, and play a video on Firefox it does not play, I have to stop music to play the video.
The specific question is: How to allow multple audio sources at the smae time, while I manually control the volume of each app in the sound control.

Update 02/11/2020: I read the Mixxx documentation, here: and it says that by default the app locks the audio in order to prevent sound latency, but they also show the workaround to start the application from the menu without locking the audio, and allowing other audio from any other app.

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