Tap the audio icon located to the bottom right of your screen, it’s the one that resembles a speaker, see below.
In the subsequent pop-up panel tap the 'mix with other apps' icon, it’s the one that looks like two crossed arrows.
Please note that when this icon is bright white (as opposed to dimmed grey) the mixer is now on, which means that Portal will remain on in the background continuing to play ambient audio while you use other music or audio apps.
It’s a great way of introducing amazing ambience when you’re listening to your favourite podcast, music, audiobooks or a guided meditation! We hope you enjoy it.