To reproduce:
- Open attached project
- Open "test" scene
- Open Audio Mixer window and select "Ambience" mixer
- Make sure that Project Browser and Audio Mixer window are next to each other and that Audio Mixer window is 'behind' or 'hidden' (see video for details)
- In Hierarchy, select "skrillex" object
- In Inspector, change the "Output" on Audio Source to something else
- Now click on the Audio Mixer tab and observe that nothing happens

Workaround: Select an object in Hierarchy view and then select Audio Mixer tab.