Apple - How can I send my computer's audio to multiple outputs?

1. Open Audio MIDI Setup from Applications > Utilities.
2. Click the + button in the bottom left corner and select "Create Multi-Output Device".
3. Check “Built-In Output” (this may already be checked) and the Soundflower output of your choice.
4. Right-click (or control-click) the device you just created and select “Use this device for sound output”.

That's where the Soundflowerbed application comes into play. It lets you select out of which output you would like to play the sound that the rest of the system is feeding into sound flower.