I contacted one of FM radio developers, and he told me that Microsoft API's don't support Radio through loud speaker.
Holy cow you are correct!
The app is misleading! When you go to the settings of the app it has a loudspeaker option, but when you click it, it takes you to a website saying it can't do it. WTF
I did some more searching around, I am a programmer too. It looks like there is an API for choosing where to output sound, but it only works with VOIP calls. However, it looks like you can tell the OS "Yeah this is a VOIP call trust me *wink* *wink*" and then it will work with generic audio.
https://blogs.msdn.microsoft.com/ma...piece-from-a-windows-phone-8-1-universal-app/
I guess radio app developers haven't done this so far. I could try this when I have time in a few weeks.
Edit: The only problem till now, though, is that W10M UWP APIs still don't have access to the radio I think. Thus, you'd have to target WP8 or something like that to be able to make an app with radio access. Maybe (hopefully) voting on user feedback will help...
feedback-hub:?contextid=375&feedbackid=e1ffe602-8561-4625-bec5-50b1f6d6af84&form=1&src=2