sending audio to external decoders
If you run Linux, you probably already know that a Virtual Audio Cable is built-tin to ALSA/Pulseaudio. For those of you running Windows, you might want to check this out.
https://www.howtogeek.com/howto/39532/how-to-enable-stereo-mix-in-windows-7-to-record-audio/
Comments
As an aside, if you run Linux you can now use Pipewire instead of Pulseaudio and Jack.
To clarify: ALSA is essentially the hardware drivers and is part of the kernel (plus a few utilities). Pulseaudio is a consumer audio server which is OK for doing one thing at a time but a pig to get to route audio between apps. Jack is a low-latency professional audio server with, as the name suggests, easy routing of audio between applications. For years many of us have used both servers with, typically, Pulseaudio routing via Jack to the hardware.
Pipewire clears out a lot of this cruft and does, essentially, everything that the two legacy servers did but in a single integrated server. It has interfaces for both sets of applications and utilities. It was still pretty new and temperamental until a few months ago but now seems very stable. I use it all the time and it's great to be able to route audio to and from Pulseaudio applications as well as Jack ones using Qjackctl.
Another nice thing it does is give separate audio outputs for each firefox tab.
Would Pipewire run on a Pi(4)? It looks like a great way to feed multiple apps with a single source.
Yes, should do. Pipewire is free software so can be built for any architecture or OS.