This has been an issue for a while. I just found a blog post from 2017 that highlights the issue and provides a "temporary workaround" recommended by Apple, (three years later)
Their solution -- which worked -- was to turn off the Airpods microphone.
Go to: Sound Preferences >> switch the sound input from your airpods to the internal microphone.
The article explains some of the technology behind the issue that causes the problem. But the reality is, this is an old problem tied to Bluetooth codecs.
The top google result for "Fixing Extremely Poor or Muffled Call Quality When Using AirPods and MacOS" explains the issue in detail.
Three... Years...
chrisgerne
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