Hello,
ABout the windows client - is it possible to play music in the background, for example with VLC? Is there any information how to use this in Xibo?
If it is online radio you can have a webpage widget that will open the radio page and play it (region with it can be very small or even outside of the layout as there is no visual representation of it).
Some radios may allow you to embed their player which should work in the embed html module in Xibo.
It could be possible to have vlc streaming audio only as well in local video module in Xibo perhaps if you can create such stream.
If you can get HLS audio stream then that can be added to HLS widget in Xibo (1.8 only).