Aggregate live microphone and music in same audio output in MacOS
Here's how you can combine the audio output of some music software (e.g. Garageband) and microphone simultaneously as the input to a streaming software (e.g. Google Meet) and your headphones. I used this setup to be able to play music from Garageband and talk in Google Meet.
1. Install Soundflower
2. Setup audio devices
Launch Applications → Utilities → Audio MIDI Setup.
First, create a Multi-Output Device. In the new device, check [x] External Headphones [x] Soundflower (64ch)
Then, create an Aggregate Device. In the new device, check [x] Microphone [x] Soundflower (64ch)
3. Select output in the music app
In your music app, e.g. Garageband, select Multi-Output Device as the Output Device.
4. Select input in the streaming app
In your Streaming app, e.g. Google Meet, select Aggregate Device as your microphone or input device. If you are using Google Meet for streaming, note that you must unselect Noise cancellation. Otherwise your music will be more or less muted.
This should do the trick.