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: