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