Filter amix
  Audio mixing.
    Inputs:
        dynamic (depending on the options)
    Outputs:
       #0: default (audio)
amix AVOptions:
  inputs            <int>        ..F.A.... Number of inputs. (from 1 to 1024) (default 2)
  duration          <int>        ..F.A.... How to determine the end-of-stream. (from 0 to 2) (default longest)
     longest         0            ..F.A.... Duration of longest input.
     shortest        1            ..F.A.... Duration of shortest input.
     first           2            ..F.A.... Duration of first input.
  dropout_transition <float>      ..F.A.... Transition time, in seconds, for volume renormalization when an input stream ends. (from 0 to INT_MAX) (default 2)
  weights           <string>     ..F.A.... Set weight for each input. (default "1 1")


