$ ./ffmpeg -i ~/write.wav -af volume=0.1 ~/lol.wav
ffmpeg version N-35540-g0cc9949, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec  7 2011 22:26:02 with gcc 4.6.1
  configuration: --enable-filter=earwax
  libavutil    51. 30. 0 / 51. 30. 0
  libavcodec   53. 41. 2 / 53. 41. 2
  libavformat  53. 24. 0 / 53. 24. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 51. 0 /  2. 51. 0
  libswscale    2.  1. 0 /  2.  1. 0
[wav @ 0x2d077e0] parser not found for codec pcm_s16le, packets or times may be invalid.
[wav @ 0x2d077e0] max_analyze_duration 5000000 reached at 5015510
Input #0, wav, from '/home/jeremy/write.wav':
  Duration: 00:04:01.86, bitrate: 1411 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, 2 channels, s16, 1411 kb/s
Unrecognized option 'af'
Failed to set value 'volume=0.1' for option 'af'