Siri stopped working
I have iPhone 5s and iOS 8.3 and my siri stopped working after replacing broken glass. My current workaround is to use the headphone microphone.

I think iPhone when using SIRI does some sophisticated background noise reduction. For that it uses additional microphones I think for iPhone 5s and up there are three all together.

Try to use the iPhone supplied Headset microphone I am certain this will work for you. This is my current workaround and it works all the time..