I have used my 2 year old AirPods simply for listening previously, but I would like to use the enhanced iOS 12 Siri.
Alas, the microphone on both AirPods does record only very muffled audio - the iOS dictation feature does not register anything, and Audio Recorder records extremely quiet audio.
Back when they were new, Siri could pick up my voice without any problem.
Things I have tried
Cleaning the microphone holes at the bottom - with a toothpick and with Q-tips and rubbing alcohol
Blowing through or sucking on the microphone holes
Using different devices - iPhone 7, iPhone Xs, MacBook Pro
Resetting the AirPods by pressing 15 seconds on the case-button