iPhone 6 Microphone Not Working

Hi all, my iPhone 6 with the latest iOS 11.4 version experienced the problem of using Siri speaker and also video call with Whatsapp and WeChat. I am still able to make voice call and audio recording. It should be an issue with the rear microphone. I have tried all the steps listed in Apple Support such as cleaning of the Rear microphone, update to latest version, re-enabled Siri, dictation, and reset network setting. I couldn't even set up Hey Siri as it did not hear me. I am sure there is nothing blocking the rear microphone. Please help if there is any further step and this occurred in a sudden when I tried to make video call with my family this afternoon. It works completely fine before.

Also, if anyone knows the price of microphone replacement? Thanks.