$ ffmpeg2 -i ~/Desktop/2mins2.mp3 -af 'asetpts=PTS-STARTPTS,afade=t=in:st=0:d=15' n.mp3

ffmpeg version N-40693-gf7bbe0f-   http://johnvansickle.com/ffmpeg/    Copyright (c) 2000-2014 the FFmpeg developers
  built on Sep  9 2014 02:20:35 with gcc 4.8 (Debian 4.8.3-10)
  configuration: --enable-gpl --enable-version3 --disable-shared --disable-debug --enable-runtime-cpudetect --enable-libmp3lame --enable-libx264 --enable-libx265 --enable-libwebp --enable-libspeex --enable-libvorbis --enable-libvpx --enable-libfreetype --enable-fontconfig --enable-libxvid --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-gray --enable-libopenjpeg --enable-libopus --disable-ffserver --enable-libass --enable-gnutls --cc=gcc-4.8
  libavutil      54.  7.100 / 54.  7.100
  libavcodec     56.  1.100 / 56.  1.100
  libavformat    56.  4.101 / 56.  4.101
  libavdevice    56.  0.100 / 56.  0.100
  libavfilter     5.  1.100 /  5.  1.100
  libswscale      3.  0.100 /  3.  0.100
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  0.100 / 53.  0.100
Input #0, mp3, from '/home/nurono/Desktop/2mins2.mp3':
  Metadata:
    encoder         : Lavf55.33.100
  Duration: 00:01:59.95, start: 0.025057, bitrate: 192 kb/s
    Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16p, 192 kb/s
File 'n.mp3' already exists. Overwrite ? [y/N] y
[afade @ 0x3718c20] Value 0.000000 for parameter 'nb_samples' out of range [1 - 2.14748e+09]
    Last message repeated 1 times
[afade @ 0x3718c20] Error setting option nb_samples to value 0.
[Parsed_afade_1 @ 0x3718b60] Error applying options to the filter.
[AVFilterGraph @ 0x36fe560] Error initializing filter 'afade' with args 'in:0:0:15'
Error opening filters!
⌂227% 1d [nurono:/home/nurono/repogitories/rutabaga] master(+62/-15) 1 ± ffmpeg2 -i ~/Desktop/2mins2.mp3 -af 'asetpts=PTS-STARTPTS,afade=in:0:15' n.mp3
ffmpeg version N-40693-gf7bbe0f-   http://johnvansickle.com/ffmpeg/    Copyright (c) 2000-2014 the FFmpeg developers
  built on Sep  9 2014 02:20:35 with gcc 4.8 (Debian 4.8.3-10)
  configuration: --enable-gpl --enable-version3 --disable-shared --disable-debug --enable-runtime-cpudetect --enable-libmp3lame --enable-libx264 --enable-libx265 --enable-libwebp --enable-libspeex --enable-libvorbis --enable-libvpx --enable-libfreetype --enable-fontconfig --enable-libxvid --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-gray --enable-libopenjpeg --enable-libopus --disable-ffserver --enable-libass --enable-gnutls --cc=gcc-4.8
  libavutil      54.  7.100 / 54.  7.100
  libavcodec     56.  1.100 / 56.  1.100
  libavformat    56.  4.101 / 56.  4.101
  libavdevice    56.  0.100 / 56.  0.100
  libavfilter     5.  1.100 /  5.  1.100
  libswscale      3.  0.100 /  3.  0.100
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  0.100 / 53.  0.100
Input #0, mp3, from '/home/nurono/Desktop/2mins2.mp3':
  Metadata:
    encoder         : Lavf55.33.100
  Duration: 00:01:59.95, start: 0.025057, bitrate: 192 kb/s
    Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16p, 192 kb/s
File 'n.mp3' already exists. Overwrite ? [y/N] y
Output #0, mp3, to 'n.mp3':
  Metadata:
    TSSE            : Lavf56.4.101
    Stream #0:0: Audio: mp3 (libmp3lame), 44100 Hz, stereo, s16p
    Metadata:
      encoder         : Lavc56.1.100 libmp3lame
Stream mapping:
  Stream #0:0 -> #0:0 (mp3 (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
[libmp3lame @ 0x3bbbbc0] Trying to remove 1152 samples, but the queue is empty
size=    1875kB time=00:01:59.95 bitrate= 128.0kbits/s
video:0kB audio:1875kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.011773%