Help requested for modifying an intercom

[parvati]

First post here, so please bear with me...
I have a marine intercom, a Raytheon430, which uses a speaker to be used as a 'talkback' system, i.e. it is also used as a microphone, for hands-free use.
I want to convert the remote talkback speaker with a press-to-talk button, so that the input from the remote is blocked until the press to talk switch is pressed, allowing privacy and stopping eavesdropping.
I simplistically though that placing a diode in the circuit would work, but it introduces too much distortion.
Any other ideas?
Thanks!

 

[ronv]

Assuming it is voice activated..... Is there an adjustment for the level where the speaker becomes the microphone? If so this might be the place to put your switch.