Theory and practicality of audio transformer...

I have a little project that I am working with that is beating me at the moment and I thought an audio transformer could help me but alas no.

From what I understand (now) is that any short circuit on the secondary side of a transformer, will be a short circuit on the primary due to induced flux across the transformer.

My problem:
I have a intercom that uses 4 wires, 2 wires for a microphone (input) and 2 wires for a speaker (output). If you short between the two pairs you get REALLY bad sounds coming out of the speaker

I need to connect a microphone (to the input) and speaker (to the output) that shares a common between the speaker and mic. I thought about using audio transformers, but I am hopeless as it turns out.

Can a person completely isolate the two? Make a circuit that will bridge between the two without creating a short circuit?

Usually, intercoms are working with a hybrid circuit at both ends, similar to an analog telephone.

Using a mic and a loudspeaker needs 3 wires - one is the common earth or screen. The problem you have is that of feedback or "singing"( post office term).When you speak into the mic, the signal goes down the line and comes out of the loudspeaker which is then picked by that mic and goes down the line again and comes out in your loudspeaker, where it gets into your microphone. . . The way to reduce this effect is to actually introduce some of the output from each loudspeaker into its microphone circuit so as to cancel out the output due to the physical acoustic coupling.
A hybrid is a special arrangement to isolate the mic and the loudspeaker signals so they can both travel down the same pair of wires.
Frank

See some previous threads about hybrid circuits:
https://www.edaboard.com/threads/218650/
https://www.edaboard.com/threads/217017/
https://www.edaboard.com/threads/8546/