$ /home/pi/special/ffmpeg/ffmpeg -re -f alsa -acodec pcm_s32le -i mic_sv test.wav
ffmpeg version git-2017-02-05-e57fd92 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.9.2 (Raspbian 4.9.2-10)
  configuration: --extra-cflags=-I/opt/vc/include/IL --enable-nonfree --enable-omx-rpi --logfile=CONFIG.TXT
  libavutil      55. 46.100 / 55. 46.100
  libavcodec     57. 75.100 / 57. 75.100
  libavformat    57. 66.101 / 57. 66.101
  libavdevice    57.  2.100 / 57.  2.100
  libavfilter     6. 73.100 /  6. 73.100
  libswscale      4.  3.101 /  4.  3.101
  libswresample   2.  4.100 /  2.  4.100
Guessed Channel Layout for Input Stream #0.0 : stereo
Input #0, alsa, from 'mic_sv':
  Duration: N/A, start: 1487309867.747179, bitrate: 3072 kb/s
    Stream #0:0: Audio: pcm_s32le, 48000 Hz, stereo, s32, 3072 kb/s
File 'test.wav' already exists. Overwrite ? [y/N] y
Output #0, wav, to 'test.wav':
  Metadata:
    ISFT            : Lavf57.66.101
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s
    Metadata:
      encoder         : Lavc57.75.100 pcm_s16le
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s32le (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
[alsa @ 0x2b5f450] ALSA buffer xrun.
[alsa @ 0x2b5f450] ALSA buffer xrun.rate=   7.8kbits/s speed=1.08x
[alsa @ 0x2b5f450] ALSA buffer xrun.rate=   7.5kbits/s speed=1.08x
[alsa @ 0x2b5f450] ALSA buffer xrun.rate=   7.4kbits/s speed=1.08x
[alsa @ 0x2b5f450] ALSA buffer xrun.rate=   7.4kbits/s speed=1.02x
[alsa @ 0x2b5f450] ALSA buffer xrun.rate=   7.4kbits/s speed=1.03x
[alsa @ 0x2b5f450] ALSA buffer xrun.rate=   7.3kbits/s speed=1.04x
size=      14kB time=00:00:15.30 bitrate=   7.4kbits/s speed= 1.1x
video:0kB audio:14kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.556507%
Exiting normally, received signal 2.