Strange problem working with sound

hi

I want to change the sound of VTS_06_1.VOB from mp2 to ac3, so I did:

/opt/new/bin/ffmpeg -i "VTS_06_1.VOB" -acodec ac3 -ab 224 -ar 48000 -ac 2 
-vcodec copy -ss 0:6:50 -vframes 3000 "VTS_06_1.copy.mpg"

the /opt/new/bin/ffmpeg is the ffmpeg that I grab & build from cvs today.

What's weired is that there is no sound in the result file (when played with
VLC). The work log and result check is as followed:

  $ /export/opt/new/bin/ffmpeg -i "VTS_06_1.VOB" -acodec ac3 -ab 224 -ar 48000 
-ac 2 -vcodec copy -ss 0:6:50 -vframes 3000 "VTS_06_1.copy.mpg"
  FFmpeg version CVS, Copyright (c) 2000-2004 Fabrice Bellard
    configuration:  --prefix=/opt/new --libdir=${prefix}/lib 
--incdir=${prefix}/include/ffmpeg --mandir=${prefix}/share/man --enable-shared 
--enable-pthreads --enable-gpl --disable-bktr --disable-dv1394 
--disable-network --disable-simple_idct --disable-ffserver 
    libavutil version: 49.0.0
    libavcodec version: 51.9.0
    libavformat version: 50.4.0
    built on Apr 22 2006 21:58:45, gcc: 3.3.5 (Debian 1:3.3.5-13)
  Input #0, mpeg, from 'VTS_06_1.VOB':
    Duration: 00:41:09.3, start: 0.280633, bitrate: 2887 kb/s
    Stream #0.0[0x1e0], 29.97 fps(r): Video: mpeg2video, yuv420p, 352x480, 2600 
kb/s
    Stream #0.1[0x1c0]: Audio: mp2, 48000 Hz, stereo, 192 kb/s
  File 'VTS_06_1.copy.mpg' already exists. Overwrite ? [y/N] y
  Output #0, mpeg, to 'VTS_06_1.copy.mpg':
    Stream #0.0, 29.97 fps(c): Video: mpeg2video, yuv420p, 352x480, q=2-31, 
2600 kb/s
    Stream #0.1: Audio: ac3, 48000 Hz, stereo, 224 kb/s
  Stream mapping:
    Stream #0.0 -> #0.0
    Stream #0.1 -> #0.1
  [mpeg @ 0xb7fc9860]buffer underflow
  [mpeg @ 0xb7fc9860]buffer underflow
  frame= 3000 q=-9099600.6 Lsize=   34702kB time=100.1 bitrate=2839.2kbits/s    
  video:31772kB audio:2738kB global headers:0kB muxing overhead 0.557089%

If checking with tcprobe, it reports no audio/video.

  $ tcprobe -i VTS_06_1.copy.mpg
  [tcprobe] MPEG program stream (PS)
  [tcprobe] summary for VTS_06_1.copy.mpg, (*) = not default, 0 = not detected
     no audio track: use "null" import module for audio

If checking with ffmpeg, it reports that the stream 0.0 and 0.1 are reversed:

  $ ffmpeg -i VTS_06_1.copy.mpg
  ffmpeg version cvs, build 4753, Copyright (c) 2000-2004 Fabrice Bellard
    configuration:  --prefix=/usr --libdir=${prefix}/lib --enable-shared 
--enable-mp3lame --host=i386-linux --enable-gpl --build=i386-linux 
--enable-faad --mandir=${prefix}/share/man --enable-vorbis --enable-pthreads 
--enable-faac --enable-xvid --enable-dts cc --enable-amr_nb --enable-amr_wb 
--enable-pp --enable-shared-pp --enable-libogg --enable-a52bin --enable-x264 
    built on Jun  9 2005 15:41:29, gcc: 3.3.5 (Debian 1:3.3.5-13)

  Seems that stream 1 comes from film source: 25.00 (25/1) -> nan (0/0)
  Input #0, mpeg, from 'VTS_06_1.copy.mpg':
    Duration: 577014:32:22.2, start: 0.500000, bitrate: N/A
    Stream #0.0: Audio: ac3, 48000 Hz, stereo, 224 kb/s
    Stream #0.1: Video: mpeg2video

What's wrong?

PS. If I use "-target dvd", then everything is ok:

/opt/new/bin/ffmpeg -i "VTS_06_1.VOB" -target dvd  -ss 0:6:50 -vframes 3000 
"VTS_06_1.dvd.mpg"

The reason that I want to specify audio parameter myself is that
- the source video is already dvd compatible
- I want to keep the original frame size, which is 352x480

$ ffmpeg -i VTS_06_1.VOB
ffmpeg version cvs, build 4753, Copyright (c) 2000-2004 Fabrice Bellard
  configuration:  --prefix=/usr --libdir=${prefix}/lib --enable-shared 
--enable-mp3lame --host=i386-linux --enable-gpl --build=i386-linux 
--enable-faad --mandir=${prefix}/share/man --enable-vorbis --enable-pthreads 
--enable-faac --enable-xvid --enable-dts cc --enable-amr_nb --enable-amr_wb 
--enable-pp --enable-shared-pp --enable-libogg --enable-a52bin --enable-x264 
  built on Jun  9 2005 15:41:29, gcc: 3.3.5 (Debian 1:3.3.5-13)
Input #0, mpeg, from 'VTS_06_1.VOB':
  Duration: 00:41:09.3, start: 0.280633, bitrate: 2887 kb/s
  Stream #0.0: Video: mpeg2video, yuv420p, 352x480, 29.97 fps, 2600 kb/s
  Stream #0.1: Audio: mp2, 48000 Hz, stereo, 192 kb/s

How can I specify my own audio parameter to covert the sound, and left the
video intact?

thanks a lot

tong