Optocoupler is not breaking circuit
The opto-coupler output is probably inappropriate for switching on the buzzer.  Many door "buzzers" run on AC line power.  The little transistor in the opto-coupler can't handle that voltage, and only works for current in one direction.

Loose the opto-coupler and use a relay instead.  The digital output won't be able to drive the relay directly.  You need a transistor in between to amplify the current.  There are many examples of how to drive relays from digital signals on this site.  Here is one example.