Hmm, I was going to recommend disabling the audio hardware in the BIOS:
hdacc0: <Realtek ALC897 HDA CODEC> at cad 0 on hdac0 hdaa0: <Realtek ALC897 Audio Function Group> at nid 1 on hdacc0 pcm0: <Realtek ALC897 (Right Analog)> at nid 20 and 24 on hdaa0 hdacc1: <Intel Kaby Lake HDA CODEC> at cad 2 on hdac0 hdaa1: <Intel Kaby Lake Audio Function Group> at nid 1 on hdacc1 pcm1: <Intel Kaby Lake (HDMI/DP 8ch)> at nid 3 on hdaa1But you probably can't do that in Coreboot.
You can see in your output though that it is booting with Video as the primary console:
Dual Console: Video Primary, Serial Secondary
If you have a serial connection I recommend setting serial as the primary console if only because it's much easier to log and copy and output from a serial terminal.