Filter volume
  Change input volume.
    Inputs:
       #0: default (audio)
    Outputs:
       #0: default (audio)
volume AVOptions:
  volume            <string>     ..F.A.... set volume adjustment expression (default "1.0")
  precision         <int>        ..F.A.... select mathematical precision (from 0 to 2) (default float)
     fixed           0            ..F.A.... select 8-bit fixed-point
     float           1            ..F.A.... select 32-bit floating-point
     double          2            ..F.A.... select 64-bit floating-point
  eval              <int>        ..F.A.... specify when to evaluate expressions (from 0 to 1) (default once)
     once            0            ..F.A.... eval volume expression once
     frame           1            ..F.A.... eval volume expression per-frame
  replaygain        <int>        ..F.A.... Apply replaygain side data when present (from 0 to 3) (default drop)
     drop            0            ..F.A.... replaygain side data is dropped
     ignore          1            ..F.A.... replaygain side data is ignored
     track           2            ..F.A.... track gain is preferred
     album           3            ..F.A.... album gain is preferred
  replaygain_preamp <double>     ..F.A.... Apply replaygain pre-amplification (from -15 to 15) (default 0)
  replaygain_noclip <boolean>    ..F.A.... Apply replaygain clipping prevention (default true)

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

