ffmpeg -i input.mp3 -af "aecho=in_gain=0.4:out_gain=0.8:delays=325:decays=0.2" output.mp3
# aecho: Indicates the name of the echo filter
# in_gain Indicates the input gain reflected signal (min: 0.1, max: 1.0)
# out_gain Indicates the output reflected signal (min: 0.1, max: 1.0)
# delays Indicates the a list of time intervals (in milliseconds) between the original signal and reflections (0.0 to 90000.0 with default 0.5)
# decays Indicates the list of loudness of each reflected signal (0.0 to 1.0 with default 0.5)