I was still running your good test kernel:
(actual kernel package appeared autoremoved) 4.15.0-66-generic #75~lp1845810 noise level low
Upgrading to bionic-updates:
4.15.0-70.79 4.15.0-70-generic #79-Ubuntu noise level high (as expected, bug not yet fixed)
Upgrading to bionic-proposed:
4.15.0-71.80 4.15.0-71-generic #80-Ubuntu noise level low (bug appears fixed)
I also checked general audio playback on the bionic-proposed kernel (4.15.0-71.80 4.15.0-71-generic #80-Ubuntu) and that also seems fine.
I was still running your good test kernel:
(actual kernel package appeared autoremoved) 4.15.0-66-generic #75~lp1845810 noise level low
Upgrading to bionic-updates:
4.15.0-70.79 4.15.0-70-generic #79-Ubuntu noise level high (as expected, bug not yet fixed)
Upgrading to bionic-proposed:
4.15.0-71.80 4.15.0-71-generic #80-Ubuntu noise level low (bug appears fixed)
I also checked general audio playback on the bionic-proposed kernel (4.15.0-71.80 4.15.0-71-generic #80-Ubuntu) and that also seems fine.