I still suspect it's an audio driver or firmware issue.
Kodi should be fine, it's the same 18.0rc3 version as in the current LE beta and except for a bunch of errors because the baytrail audio device doesn't support surround the kodi log looks fine (at least for opening the audio device).
One difference between kodi and speaker-test though is that kodi outputs audio in 24bit format whereas speaker-test uses 16bit.
An interesting test would be playing a 24bit 48kHz stereo WAV file with aplay. If there's a driver / firmware bug it should give the same (or similar) distortions as kodi.
Can you also test the ssp0 and windows driver firmware files and the amixer settings as I outlined before?
so long,
Hias