I believe this issue has been present since iOS 8, but I personally didn't experience it until iOS 9. My friends and I often use the Messages app to send voice messages. For some reason, I receive the first voice message as a proper iMessage...but then the same message shows up again a few minutes later as an SMS message. I believe it may have something to do with Text Message Forwarding on my iMac, but I'm not sure. Anyone else experiencing this?