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You can use the serial TX bit to drive an optoisolator, which then triggers a separately powered buzzer or bell:

serial TX bit                12vDC power supply
__________                     ^
          \                    |
           |                   \__
           |                      @
           |                      @ buzzer coil or relay
           |                      @
           |                    --
           |                   /
           |                   |
           |                   /
opto LED   V  /\/\light/\/\/\ | opto photodiode
          ---                  \
           |                    v
           /                    |
sio gnd ---                     |
                                |
                               ---
                                =  gnd
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Depending on the rating of your optoisolator, you might want to put a zener voltage-limiter in parallel with it. Most any opto should work. Just send bits out the serial port to trigger it. Your remote webserver can send a signal to your office machine through a socket to a daemon, which then talks to the port. Different char strings will cause the coil to energize by different amounts. :D Alternately, you can trigger one of the SIO control bits (as I did in the original article) for better control. Sys:PortIO should be helpful.

UPDATE: I should have separated the grounds to be truly safe: Ground from the TX pin should go to the gound of the SIO connector (DB-9 or DB-25), and ground of the power driver side should go to the ground of the DC side of the power supply. Sorry about that!