The micmute key needs to be remapped to F20 for userspace to consume it.
See https://gitlab.gnome.org/GNOME/gnome-settings-daemon/issues/121
evdev:name:Asus Laptop extra buttons:dmi:bvn*:bvr*:bd*:svnASUS*:pn*:pvr*
KEYBOARD_KEY_6b=f21 # Touchpad Toggle
+# USB keyboard in Asus FX503VD
+evdev:input:b0003v0B05p1869*
+ KEYBOARD_KEY_ff31007c=f20 # Remap micmute to f20
+
###########################################################
# BenQ
###########################################################