SYS AS SYSDBA> -- You must commit when you are finished to add the line to dba_jobs.
SYS AS SYSDBA> -- You must run this as sys to get the job to have the right path and privs.
SYS AS SYSDBA> variable jobno number;
SYS AS SYSDBA> variable instno number;
SYS AS SYSDBA> begin
  2    select max(job)+1 into :jobno from dba_jobs;
  3    select instance_number into :instno from v$instance;
  4    dbms_job.submit(:jobno,
  5  '
  6  begin
  7  delete from sys.aud$ where trunc(NTIMESTAMP# ) < trunc(sysdate -7);
  8  end;
  9  ',
 10  trunc(sysdate)+1+1/24/60, 'trunc(sysdate)+1+1/24/60',
 11   TRUE, :instno);
 12  end;
 13  /

PL/SQL procedure successfully completed.

SYS AS SYSDBA> commit;

Commit complete.

SYS AS SYSDBA> select job,to_char(next_date,'DD-MON-RR HH24:MI') next_date,
  2  what,interval,SCHEMA_USER,instance,broken from dba_jobs
  3  where upper(what) like '%AUD$%';

  JOB NEXT_DATE       WHAT
----- --------------- --------------------------------------------------
INTERVAL
---------------------------------------------------------------------------
SCHEMA_USER     INSTANCE B
--------------- -------- -
    4 05-JUL-12 00:01
                      begin
                      delete from sys.aud$ where trunc(NTIMESTAMP# ) < t
                      runc(sysdate -7);
                      end;
trunc(sysdate)+1+1/24/60
SYS                    1 N