Playing music whilst using Facetime

For a few weeks, I was able to speak to someone using Facetime (with video paused) and at the same time call up a song on Spotify and the person at the other end was able to hear it, (as was I through the earphones). Now, for some reason, They no longer hear the song, though I can. (By holding a separate iPod to the microphone I can accomplish this; however, it was so much simpler when I could do it on one device.)

I can't think of anything that changed to stop that clever trick, unless it was an iOS upgrade, but if anyone has any thoughts I'd be grateful.