https://blogs.oracle.com/imc/entry/oracle_database_12c_new_unified

Enalbe unified audit

1.Stop all Oracle processes: databases, listener and Enterprise Manager.
2.Relink Oracle with the uniaud_on option.

cd $ORACLE_HOME/rdbms/lib
make -f ins_rdbms.mk uniaud_on ioracle

[root@oel7-2 ~]# . oraenv
ORACLE_SID = [root] ? +ASM
[root@oel7-2 ~]# srvctl stop database -d orcl
[root@oel7-2 ~]# srvctl stop asm -f
[root@oel7-2 ~]# su - oracle
[oracle@oel7-2 ~]$ cd $ORACLE_HOME/rdbms/lib
[oracle@oel7-2 lib]$ make -f ins_rdbms.mk uniaud_on ioracle
..
[oracle@oel7-2 lib]$ logout
[root@oel7-2 ~]# srvctl start database -d orcl

check that option is enabled:

23:07:24 (1)[PDBORCL]c##bushmelev_aa@orcl>  select * from v$option where PARAMETER = 'Unified Auditing'
23:07:29   2  ;

PARAMETER                                          | VALUE      |     CON_ID
-------------------------------------------------- | ---------- | ----------
Unified Auditing                                   | TRUE       |          0