Nice Audio Mixer/Repeater

Solution 1:

Soundflower from Cycling74 used to be the app to do this type of task, but is unsupported & no longer reliable - I eventually had to uninstall it.

It was taken over by Rogue Amoeba, who maintain the download but no longer provide updates. It has been open sourced on GitHub, but I've yet to see a stable release; kernel panics are frequent.

It seems quite possible they re-worked the technology into their main product, Audio Hijack. Although primarily marketed as an 'any app' recorder, it actually can achieve some pretty comprehensive routing; comes with a fair number of built-in audio effects & can also use any AU plugin effects too.

This is a quick example, using 2 apps, EQ, volume, optional recorder & a specific audio output.

...and another with 3 sources to 3 outputs...

If you need even more comprehensive routing, there's always Plogue Bidule... if you can ever figure it out ;-)