AirPods Poor Siri Voice Quality

OP here – I'd just like to add to the answer below, that I've been in contact with Apple Support.

Explanation

Apple claims that the poor Mono 8kHz quality which affects recording and indeed simultaneously playback on Mac when the AirPod microphones are activated, is because the SCO codec then gets employed over the entire Mac audio system. This is supposedly "expected behaviour" when trying to use the AirPods and other Bluetooth headsets together with a computer, according to Apple.

The AAC codec is normally used when just listening to playback on the AirPods. It's just very unfortunate that SCO – low-quality as it may be – upon AirPod microphone activation is not only limited to doing recording, but also displaces AAC and audio playback.

Apple Support claims that Apple is looking at this issue, and that improvements might be coming in future firmware updates, but I did not interpret that as a promise to be honest. But for the time being, I'd say that the benefits of making calls and so forth with AirPods on Macs are quite limited.

Current best practice

Current best practice is to use the Internal Microphone for Recording, and the AirPods for playback when conducting calls. Input and Output devices can be selected in System Settings > Sound or by ⌥-clicking the Volume icon in the macOS Menu Bar, and selecting devices there.

See the screenshots below:

I couldn't do it in iMovie either (just as you show in your screenshots), but I have an idea.

Just record the audio using Quicktime Player.

Open Quicktime Player and select "New Audio Recording." Your Airpods should show up there as an audio source.

To time the audio with your video, start recording, then switch to iMovie. Hit play on the video timeline (ensure you have no sound playing) and talk along with the video.

Then take your resulting recording from Quicktime Player, pull it into iMovie, lay it on your timeline and sync it up. (Obviously you'll want to trim the ends off.)

(There are alternatives, such as using Soundflower to create patches from one audio source to another, but that seemed a lot more convoluted and complicated to go into.)