fails

ffmpeg -i sintel.mpg -f flv  -ar 44100 -an -y test.m4v

results in this file, which has no video in WMP.

c:\vids>c:\installs\ffmpeg-20120622-git-c17808c-win32-static\ffmpeg-20120622-git-c17808c-win32-static\bin\ffplay test.m4v
ffplay version N-41843-gc17808c Copyright (c) 2003-2012 the FFmpeg developers
  built on Jun 22 2012 17:40:07 with gcc 4.7.1
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-runtime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass --enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopenjpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil      51. 61.100 / 51. 61.100
  libavcodec     54. 29.100 / 54. 29.100
  libavformat    54. 11.100 / 54. 11.100
  libavdevice    54.  0.100 / 54.  0.100
  libavfilter     2. 82.100 /  2. 82.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, flv, from 'test.m4v':
  Metadata:
    encoder         : Lavf54.31.100
  Duration: 00:00:59.12, start: 0.000000, bitrate: 561 kb/s
    Stream #0:0: Video: flv1, yuv420p, 720x480, 200 kb/s, 29.97 tbr, 1k tbn, 1k tbc
[ffplay_buffer @ 03e987e0] w:720 h:480 pixfmt:yuv420p tb:1/1000 fr:0/1 sar:0/1 sws_param:
  10.64 A-V:  0.000 fd=   0 aq=    0K

(same with avi)

works

ffmpeg -i sintel.mpg -vcodec libx264 -an yo.m4v

c:\vids>C:\installs\ffmpeg-distro-static-2012-10-10-42212c664c24d813ff5d843dd04aac5cf6d7de45.frei0r\ffmpeg-distro-static-2012-10-10-42212c664c24d813ff5d843dd04aac5cf6d7de45\ffmpeg-32 -i sintel.mpg -vcodec libx264 -an yo.m4v
ffmpeg version N-45279-g6b86dd5 Copyright (c) 2000-2012 the FFmpeg developers
  built on Oct 10 2012 17:30:47 with gcc 4.7.1 (GCC)
  configuration: --enable-memalign-hack --arch=x86 --enable-gpl --enable-libx264 --enable-avisynth --enable-libxvid --target-os=mingw32 --cross-prefix=/home/rogerdpack/dev/ffmpeg-windows-build-helpers/sandbox/mingw-w64-i686/bin/i686-w64-mingw32- --pkg-config=pkg-config --enable-libmp3lame --enable-version3 --enable-libvpx --extra-libs=-lws2_32 --extra-libs=-lpthread --enable-zlib --extra-libs=-lwinmm --extra-libs=-lgdi32 --enable-librtmp --enable-libvorbis --enable-libtheora --enable-libspeex --enable-libopenjpeg --enable-gnutls --enable-libgsm --enable-libfreetype --disable-optimizations --enable-mmx --disable-postproc --enable-fontconfig --enable-libass --enable-libutvideo --enable-libopus
 --disable-w32threads --extra-cflags=-DPTW32_STATIC_LIB --enable-frei0r --enable-filter=frei0r --enable-libvo-aacenc --enable-runtime-cpudetect
  libavutil      51. 74.100 / 51. 74.100
  libavcodec     54. 65.100 / 54. 65.100
  libavformat    54. 31.100 / 54. 31.100
  libavdevice    54.  3.100 / 54.  3.100
  libavfilter     3. 19.102 /  3. 19.102
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 16.100 /  0. 16.100
[mpeg2video @ 03B8F8E0] ac-tex damaged at 40 7
[mpeg2video @ 03B8F8E0] Warning MVs not available
[mpeg2video @ 03B8F8E0] concealing 1035 DC, 1035 AC, 1035 MV errors in I frame
[mpeg @ 03B8F3E0] max_analyze_duration 5000000 reached at 5024000
Input #0, mpeg, from 'sintel.mpg':
  Duration: 00:14:47.58, start: 0.233367, bitrate: 4132 kb/s
    Stream #0:0[0x1e0]: Video: mpeg2video (Main), yuv420p, 720x480 [SAR 32:27 DAR 16:9], 7000 kb/s, 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc

    Stream #0:1[0x80]: Audio: ac3, 48000 Hz, 5.1(side), s16, 448 kb/s
File 'yo.m4v' already exists. Overwrite ? [y/N] y
using SAR=32/27
[libx264 @ 03C4AA40] using cpu capabilities: MMX2 SSE2Fast SSSE3 FastShuffle SSE4.2 AVX
[libx264 @ 03C4AA40] profile High, level 3.0
[libx264 @ 03C4AA40] 264 - core 128 r2216 198a7ea - H.264/MPEG-4 AVC codec - Copyleft 2003-2012 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, ipod, to 'yo.m4v':
  Metadata:
    encoder         : Lavf54.31.100
    Stream #0:0: Video: h264 (avc1 / 0x31637661), yuv420p, 720x480 [SAR 32:27 DAR 16:9], q=-1--1, 30k tbn, 29.97 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mpeg2video -> libx264)
Press [q] to stop, [?] for help
[mpeg2video @ 03B8F8E0] ac-tex damaged at 40 7
[mpeg2video @ 03B8F8E0] Warning MVs not available
[mpeg2video @ 03B8F8E0] concealing 1035 DC, 1035 AC, 1035 MV errors in I frame
frame=  560 fps=146 q=-1.0 Lsize=    1408kB time=00:00:18.61 bitrate= 619.6kbits/s dup=49 drop=0
video:1402kB audio:0kB subtitle:0 global headers:0kB muxing overhead 0.463819%