After the install of MacOS Mojave Developer Beta 3 I have a very poor sound thorugh my AirPods. The sound is able to compare with if you launch Siri on Mac and listen to music directly after closing Siri on MacOS. I don't has this problem on my iPhone running iOS 12 Developer Beta 3.
AirPods poor sound
Are the AirPods selected as audio input? Hold down the option key and click the speaker in the menu bar. Should be able to select another input source there and see if that fixes it.