I repeat only the last word you say. The more I repeat, the softer I got. I cannot be seen but can be heard. What am I?

Why this works

This riddle cleverly plays with the characteristics of an echo. An echo repeats the last sounds or words you say, which corresponds to the phrase "I repeat only the last word you say." The second part, "the more I repeat, the softer I got," refers to how echoes fade and become quieter with each repetition. Finally, echoes are auditory phenomena; you can't see them, but you can definitely hear them, making "an echo" the perfect answer!