Final answer:
The method that conceals information in an audio file by adding extra sound to an echo is called Echo hiding, not Frequency modulation or Amplitude modulation.
Step-by-step explanation:
The method of Audio Steganography in question, which conceals information by adding an extra sound to an echo within an audio file, is known as Echo hiding. Neither Frequency modulation (FM) nor Amplitude modulation (AM) is the correct answer.
Echo hiding is a technique where information is hidden within the echo of an audio file. Unlike Amplitude modulation, which involves varying the amplitude of a carrier wave while maintaining constant frequency to encode information, echo hiding embeds additional sounds into the natural delays and reverberations in the audio to conceal data.