Enabling audit in Oracle
I know I'm late for party, but someone else with similiar issue may find it useful so anyways: I would say that you don't have AUDIT_TRAIL set to DB,EXTENDED but perhaps only to DB or something.

If you want SQLTEXT to be populated it is mandatory to have AUDIT_TRAIL set to DB,EXTENDED. 

Check your current AUDIT_TRAIL status:

select * from v$parameter where name like '%trail%';


If it's not set to DB,EXTENDED, you set it with the following command:

alter system set audit_trail=db,extended scope=spfile;


Bear in mind that after setting it to DB,EXTENDED, you have to restart your instance for changes to take effect.

Afterwards after each audit record your SQLTEXT in SYS.AUD$ will also be populated.

From Oracle's documentation you will see more:
https://docs.oracle.com/cd/B19306_01/server.102/b14237/initparams016.htm#REFRN10006