Thanks @osss.
I indeed use it with an internet radio stream. Though at this moment you have to select the radio stream manually for the used zone. So if the alarm expires and there is nothing to play you will be 
in silence.
My plan is to support preselection of internet radio, playlist, etc. but I still have to figure out what is possible with the Roon API in this respect. Also I want to provide a first test version to the community before implementing this new functionality. I think that in its current state the Alarm Clock can be of use to more people than just me. This will also provide me some valuable feedback.