Someone brought this to my attention...

Is there not an application to do this already? It's pretty easy to do .

Not really a bug. The radio audio on the N900 comes from the same "line-in" where the microphone is connected, so C FM Radio keeps pumping data to the speakers, even if exiting QRadio turned the actual radio off.

In fact from command line running "pacat -r | pacat -p" should do the trick too, if you install "pulseaudio-utils" from apt-get.