The onboard audio device has the same Vendor and Product ID [1458:a0b8] as the audio device on motherboard GA-AX370 Gaming 5 mentioned in this forum discussion: No sound after upgrade
The device has a kernel patch for the dual codecs
ALSA: hda - Support Gigabyte Gaming board with dual Realtek codecs - Patchwork
The patch mentions a Use Case Manager configuration
check if package alsa-ucm-conf is installed to provide the UCM configuration file /usr/share/alsa/ucm2/HDA-Intel/HDAudio-Gigabyte-ALC1220DualCodecs.conf
The Lei Mao blogpost ‘Successful Solution’ was to edit file /usr/share/pulseaudio/alsa-mixer/paths/analog-output-lineout.conf
But the mixer path configuration files are in a different location on Manjaro
The configuration file from package alsa-card-profiles is located at /usr/share/alsa-card-profile/mixer/paths/analog-output-lineout.conf