Mic not working for speakerphone, Siri and dictation

My iPhone5 mic works fine (handset mode cellular or FaceTime/ whatsapp/ Skype, voice memo, video recording etc.) except for dictation, Siri and speakerphone (any speakerphone call-cellular or VoIP). It appears that all three mics are working fine and no hardware issue is there. I tried it by whispering near each of the mic in voice memo and all the recording was audible. Google now works fine as well. I have tried restoring phone software by iTunes and using it as new instead of restoring old backup. Even this doesn't solve the problem. I think my phone's mic stopped working while using speakerphone, Siri and dictation since I updated to iOS 8.3. Any known bug in it?

How to fix the problem?