Thanks - I see.
I noticed that I can get the event in hammerspoon (http://www.hammerspoon.org/) which I am using (via hs.screen.watcher) but had hoped it would be easier in Alfred.
So I have to find a way, either to bridge from the hammerspoon event to alfred, or actually make the whole thing in hammerspoon (which should be possible through the hs.audiodevice module. I guess in the end it might be easier to do it here.
Thanks in any case!
Axel