iphone 6s mic not working on some apps
So i bought a used iphone 6s i set it up update it to the latest ios and when i go to record a snapchat video or a voice message on messenger or record a voice memo the mic doesnt pick up anything but when using siri or recording video with both front and rear facing camera the mic works and i have sound it also works when i get calls.I have tried almost everything people suggest on forums still no luck.If you know something about this problem or know how to fix it please help.Thanks
iPhone 6s, iOS 11.2.6