I have an iPhone 8 and am experiencing the exact same issue. My software is up to date.
My mic works fine with Voice Memos and with text dictation, and I’ve restarted my phone. But when I hold or press the messages audio recording icon, the recording button turns on and I see 0:00, but I speak and nothing gets recorded and the 0:00 never progresses to 0:01.
I can’t attach a video here, but I did a 20-second screen recording to show how nothing gets recorded when I’m speaking. Here’s a screenshot showing the view that just stays even when I’m speaking. 