[FFmpeg-user] audio failure on spliced mpeg2 video

[Beeblebrox]

I have a large video (3.1G) and I need to run several conversion tests in
order to solve another issue. So I decided to take a smaller slice of this
video for the test runs:
$ ffmpeg -i day7.mpg -ss 00:00:00 -t 00:04:00 t.mpg
This turned out fairly normal, but I wanted a sample containing more
frequency ranges (the video is human speech and was created by
concatenating several smaller *.mpg videos into this larger file).
The next run resulted in normal video, BUT audio that has almost only
static. The human speech can be heard as "background sound". I also wonder
if this problem could be related to my other issue. What is the reason for
this problem?
$ ffmpeg -i day7.mpg -ss 00:03:00 -t 00:07:00 t.mpg
ffmpeg version 2.1.1 Copyright (c) 2000-2013 the FFmpeg developers
  built on May  7 2014 12:00:29 with FreeBSD clang version 3.4
(tags/RELEASE_34/final 197956) 20140216
  configuration: --disable-libaacplus --disable-indev=alsa
--disable-outdev=alsa --enable-libopencore-amrnb --enable-libopencore-amrwb
--disable-libass --enable-libcdio --disable-libcelt --disable-libfaac
--enable-libfdk-aac --enable-ffserver --enable-fontconfig
--enable-libfreetype --enable-frei0r --disable-gnutls --disable-libgsm
--enable-iconv --disable-libmp3lame --disable-libbluray --enable-libv4l2
--disable-libmodplug --disable-openal --disable-indev=openal
--enable-libopencv --enable-libopenjpeg --disable-libopus --enable-libpulse
--disable-librtmp --enable-libschroedinger --disable-libspeex
--enable-libtheora --enable-vaapi --enable-vdpau --enable-libvorbis
--disable-libvo-aacenc --disable-libvo-amrwbenc --enable-libvpx
--enable-libx264 --enable-libxvid --enable-x11grab --prefix=/usr/local
--mandir=/usr/local/man --datadir=/usr/local/share/ffmpeg --enable-shared
--enable-gpl --enable-postproc --enable-avfilter --enable-avresample
--enable-pthreads --enable-memalign-hack --disable-libstagefright-h264
--disable-libutvideo --disable-libsoxr --cc=cc --extra-cflags='-msse
-I/usr/local/include/vorbis -I/usr/local/include'
--extra-ldflags='-L/usr/local/lib ' --extra-libs=-pthread --disable-debug
--enable-nonfree --enable-version3
  libavutil      52. 48.101 / 52. 48.101
  libavcodec     55. 39.101 / 55. 39.101
  libavformat    55. 19.104 / 55. 19.104
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 90.100 /  3. 90.100
  libavresample   1.  1.  0 /  1.  1.  0
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Input #0, mpeg, from 'day7.mpg':
  Duration: 00:00:24.81, start: 0.226767, bitrate: 1070983 kb/s
    Stream #0:0[0x80]: Audio: ac3, 48000 Hz, stereo, fltp, 256 kb/s
    Stream #0:1[0x1e0]: Video: mpeg2video (Main), yuv420p(tv), 720x576 [SAR
64:45 DAR 16:9], max. 9100 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc
File 't.mpg' already exists. Overwrite ? [y/N] y
[mpeg @ 0x80cc3ea20] VBV buffer size not set, muxing may fail
Output #0, mpeg, to 't.mpg':
  Metadata:
    encoder         : Lavf55.19.104
    Stream #0:0: Video: mpeg1video, yuv420p, 720x576 [SAR 64:45 DAR 16:9],
q=2-31, 200 kb/s, 90k tbn, 25 tbc
    Stream #0:1: Audio: mp2, 48000 Hz, stereo, s16, 128 kb/s
Stream mapping:
  Stream #0:1 -> #0:0 (mpeg2video -> mpeg1video)
  Stream #0:0 -> #0:1 (ac3 -> mp2)
Press [q] to stop, [?] for help
frame=10500 fps=275 q=31.0 Lsize=   19226kB time=00:06:59.98 bitrate=
375.0kbits/s
video:12495kB audio:6562kB subtitle:0 global headers:0kB muxing overhead
0.885386%

-- 
FreeBSD_amd64_11-Current_RadeonKMS