"Built-in Mic Not Active" iPhone 5

Hey there I have an iPhone 5 that has a fault with the mic and speaker while in a phone call and video's ect.. I have tried changing out all the hardware (put the logic board in a perfect working order iphone5) which made no difference, which suggests its a problem with the logic board or a software problem.

When trying to activate siri I receive a message titled "Built-in Mic Not Active"

Was wandering if anyone had come across this before or knew a fix for this issue.

Many thanks,

Chris