I wondered if it might be possible to send sound output from my teeny Macbook to both a USB-C display and a bluetooth headset simultaneously, with different volume setpoints. Turns out, it’s extremely straightforward!
- Open Audio MIDI Setup app. It is located in the Utilities subfolder of Applications.
- Click on the + at bottom left to create a new Multi-Output[/Aggregate] device. Name it.
- Click on the device name in the list at left, and then select Use for each input/output source you want audio sent to[/accepted from]. Volume levels can be set differently for each source and channel by clicking on that device in the list at left.
- If you want to control the master volume for these combo devices with your keyboard, get Soundflower and follow these instructions, which work even with Multi-Output/Aggregate devices.