Blackhole lets you route audio.

https://existential.audio/blackhole/

I’m not aware of anything free - but would be excited if there were. For the Mac, I use Loopback https://rogueamoeba.com/loopback/

And it gives me complete control over what audio I send. One example is that when I want to share a video I can share my screen and the audio streams to those who are watching with me.

Also it helps when I want to play some sound like a sound board.

On Windows, VoiceMeeter may do what you need.

Of course, OBS also mixes audio and if you add a loopback audio device and set it as output in OBS, you can also use that as an input elsewhere. On Windows or macOS, you can use VB Audio Cable. On Linux, it should be pretty straight forward with most audio stacks, as well.

Equalizer APO: https://sourceforge.net/projects/equalizerapo/

Runs audio filters at the Windows driver level, before applications get the data. Good for adding noise reduction etc. to applications that don't support it natively.

Depends on what you're looking for. I know on Linux it's pretty straightforward to pipe any audio source into another, so for example you can send music coming out of your own Zoom microphone.