Whether you can use an fm radio card depends on how you want to use it
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If you don't want to see your radio card software but just want to use buttons from within xlobby to change settings on your radio, well....... it's hard and limited. Some program's are available which can change the frequency of the card. THose can also be instruced from an other program. But you can not get any info from the radio card back for display in xlobby. You canj seach in myhtpc forums. Some people are working very hard on this item.
So I have taken a different approach. When I click on the radio button, xlobby goes to an empty screen and then opens my radio app (terratec cynergy 600). This way I also have the RDS info on my screen.
The only problem which remaind was that if I went to a different screen, let's say pictures, and back to radio, the radio app was behind xlobby. Setting focus on the radio can be done with a little program, I think it is called setfocus.exe, just search this forum. I use girder, but I think it can also be done using recent xlobby versions. I you like, I can send you the girder gml file.