I have an app on an iPad that needs to request access to the microphone. I think when the initial prompt came up the kids said "No". Now, the app requesting the microphone is not showing up in Settings--> Privacy--> Microphone. Actually, there are no apps in the microphone settings.
This is way old, but I am working on a microphone/app issue today with on iOS11 ipad (dictation on/Siri off in config profile). If you go to settings and click on the app itself, some of my apps have slider bars for whether or not they were allowed to access the microphone.
Anyway, that's one more place to look. Really hoping either that or just uninstalling/reinstalling the app does the trick, because I don't want to turn Siri on if I can help it.