Ever since I purchased the AirPods (1st Generation), they demonstrate extremely poor quality while activating the AirPod mic, whilst they are connected to a Mac. When activating the mic to record audio, it seems that both the recording quality AND audio playback quality severely drops to a substandard level (Mono 8.0 kHz).
For example, if I invoke Siri on the Mac, whatever is being said and recorded by me, AND the Siri/Mac/AirPod audio response will be in poor quality. After the recording is done, audio playback quality usually goes back to normal1. Any mic audio recorded to a file will however be in the poor quality; which is why I interpret the issue to affect both recording and playback qualities.
The issue appears to be identical to the one discussed in this Apple Support Forum thread. The thread offers possible explanations but no definitive solution or suggested course of action.
I have this issue with several Macs of various recent models. I don't have this problem when using the AirPods with an iPhone. I have tried resetting the Mac Bluetooth interface, tried resetting the AirPods, and so forth.
FYI: Using the macOS native application Audio MIDI Setup, I attempted to observe what's happening when AirPod recording is activated. Output quality drops from the standard 2 ch 24-bit Integer 44.1 kHz → 1 ch 16-bit Integer 8.0 kHz.
1It has happened that the Mac got permanently stuck in low quality until system reboot, or even whenever the AirPods were connected afterwards. I had to attempt AirPod unpair/re-pair, and even a charging case reset to fix the latter situation.
Best Answer
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: