- In Indigo, have it speak some text. It doesn't matter what or how, just having some speech will cause the hidden process that actually speaks to launch.
- Switch to Airfoil
- Hold down the Option key and select the application source menu at the top of the Airfoil window
- You'll see a huge list of every process that's running on your Mac - search and select the "com.apple.speech.speechsysthesisd" item
That's it. Airfoil will now direct all speech to whatever speakers you have selected. In Indigo, you should see that in the Application sources menu when you do a Change Audio Source action.