Looking at a few loopback apps, capturing the audio stream from a specific application seems to be possible. Would it be a lot of effort to get this feature in Vuo as well? Without an external audio interface, capturing/loopback interfaces are by far the biggest pita when it comes to implementing audio reactiveness in the compositions. I usually hook up an external device when performing with the compositions, but for programming I'm just too lazy to set it up/don't have it at home (it is also difficult enough balancing the laptop on my lap when travelling by boat).
This will analyse the ambient loudness and smooth the sensitivity of the audio input over some seconds to allow the composition to look nice (lights not to be too small or too big) and analyse the frequencies wether the ambient volume is high or low. May be better methods to do so, any other method welcome.