AMD support for the Renoir Audio Co-processor was only released in June 2020
Linux 5.8 Sound Changes Bring New AMD Renoir ACP Driver - phoronix.com
The built-in digital microphone array (DMIC) has the new driver snd_acp3x_rn correctly loaded
but probably also needs a later version of Pulseaudio
[SOLVED] Microphone not working on Thinkpad T14s / Arch Linux Forums
suggest you install pulseaudio-git from AUR
Another Manjaro user posted recently that they had to remove packages
libao, fluidsynth, lib32-fluidsynth and manjaro-pulse to get pulseaudio-git installed
and then reinstall those packages
HP Envy - Sound does not work in Manjaro 20.1 PLASMA
(but manjaro-pulse is only a metapackage so installing it again is not really necessary)