Interesting as the behavior reprodcuces in 9i with an anonymous block

SQL> drop table t1
2 /

Table dropped.

SQL> create table t1 (c int)
2 /

Table created.

SQL> audit all on t1 by session
2 /

Audit succeeded.

SQL> insert into t1 values(1)
2 /

1 row created.

SQL> commit
2 /

Commit complete.

SQL> select userid,to_char(timestamp#,'hh24:mi:ss') audit_time from sys.aud$
2 /

USERID AUDIT_TIME
-------------------- ------------------------------
STMONTGO 21:17:16

SQL> begin
2 execute immediate ('insert into t1 values(2)');
3 end;
4 /

PL/SQL procedure successfully completed.

SQL> select userid,to_char(timestamp#,'hh24:mi:ss') audit_time from sys.aud$
2 /

USERID AUDIT_TIME
-------------------- ------------------------------
STMONTGO 21:17:16

SQL> select * from t1;

C
----------
1
2