MacOS does not come natively with internal audio routing; you will need to download a third party application in order to accomplish this. These programs allow you to capture the computer's internal audio stream and use it in applications like OBS. For computers running 10.9 (Mavericks) to 11 (Big Sur), we recommend using BlackHole from Existential Audio. If you are using a different audio routing application, the process of connecting it to your computer's internal audio stream will be similar.
In this guide:
You can download BlackHole and find complete installation instructions here.
Setting Up A Multi-Output Device with BlackHole
Now that BlackHole has been installed, it needs to be paired with an audio output. We do this with a Multi-Output Device.
Naming the source can be useful to organizing your sources, but it is not necessary. We have named this source 'Computer Audio'.
Aggregate Devices can be used in place of a Multi-Output device when configuring I/O settings in a DAW that only allows one I/O Device. Aggregate Devices are also used when combining the I/O of multiple interfaces.
In order for your Aggregate Device to properly feed audio to OBS, BlackHole must be set as Output Channels 1 & 2. If it is not, select the BlackHole item beneath the Subdevices header and drag it to the leftmost position.
Play audio through the device and check OBS to confirm that your Aggregate Device is functioning correctly.
Note: If you have already set up your Multi-Output Device to work in OBS, since it uses BlackHole as its source, you should see the signal come through the same source.