Implement Module-Loopback
There is a soundboard for linux called Soundux (https://github.com/Soundux/Soundux). It is available as a flatpak. It is able to play audio to any application that accepts audio input, and works fine with regular pulseaudio, but not with pipewire-pulseaudio.
It is supposed to be able to detect running programs to allow it to play sounds to them but it doesn't seem to detect anything when running pipewire-pulseaudio. I don't know if this is a soundux issue, or a pipewire issue. Please note I am not a developer so I don't know how this works.
I am using Fedora 33 KDE, on X11 with the Soundux flatpak from flathub and pipewire 0.3.19 from updates-testing repos.