Since upgrading to IOS 11.3.1 last week Siri can't understand me, also dictation is REALLY bad and speaking slowly and loudly does no good . . . here is what I found: SIRI uses the front mic at the top (by the earpiece), I tried taking a selfie video and found that for the first few seconds of the video the mic was dead, then it suddenly worked, I tried again and had the same thing each time I took a new selfie video, random muted mic, sometimes for as long as 5-6 seconds, sometimes cutting in and out. If Siri and dictation can't understand you, perhaps you could also try taking a selfie video because that uses the same mic as SIRI and dictation, see if your mic works . . . . . I have an iPhone 7 Plus, my other IOS devices are OK.
