From the owner's manual:
& Selecting audible signal operation
Using an electronic chirp, the system will
give you an audible signal when the doors
lock and unlock. If desired, you may turn
the audible signal off.
Perform the following steps to deactivate
the audible signal. You can also use the
same steps to restore the function.
1. Sit in the driver’s seat and shut all
doors and the rear gate.
2. Hold down the REAR (UNLOCK) side
of the power door locking switch.
3. While holding down the REAR (UNLOCK) side of the power door locking
switch, pull the key out and re-insert it into
the ignition switch at least 6 times within
10 seconds after Step 2.
4. Open and close the driver’s door once
within 10 seconds after Step 3.
5. The turn signal lights flash 3 times to
indicate completion of the setting.
You may have the above settings done by
your SUBARU dealer. Also, for models
with a multi function display, the setting
can be changed using the display. For
details, refer to “Multi function display
(Non-US vehicles only; if equipped)