[TheLoafOfBread]

Try to demodulate QAM64 without knowing what it is. It will look just like little bit elevated noise. Now imagine that aliens will be using as effective as possible modulation. There is close to zero chance to figure out what is being transferred.

Furthermore it also makes no sense to just scream in all directions so you will use highly directed microwave beam or laser. If you are not in the path of such communication, you will hear silence.