Filter apad
  Pad audio with silence.
    Inputs:
       #0: default (audio)
    Outputs:
       #0: default (audio)
apad AVOptions:
  packet_size       <int>        ..F.A.... set silence packet size (from 0 to INT_MAX) (default 4096)
  pad_len           <int64>      ..F.A.... set number of samples of silence to add (from -1 to I64_MAX) (default -1)
  whole_len         <int64>      ..F.A.... set minimum target number of samples in the audio stream (from -1 to I64_MAX) (default -1)

This filter has support for timeline through the 'enable' option.

