Why doesn't MariaDB server recognise Server Audit settings in config file?
From the documentation:


  Although the plugin's shared library is distributed with MariaDB by
  default, the plugin is not actually installed by MariaDB by default.


So you need to install the plugin - either execute:

INSTALL SONAME 'server_audit';


Or put it in your option file:

[mariadb]
...
plugin_load_add = server_audit