root@nwss-i3-01:~/scott# ffmpeg -i out.avi -c:v copy -c:a libfaac -b:a 128k out.mp4
ffmpeg version git-2013-03-28-dd17843 Copyright (c) 2000-2013 the FFmpeg developers
  built on Mar 28 2013 15:00:45 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-shared --enable-gpl --enable-nonfree --enable-libfaac --enable-libmp3lame --enable-libtheora --enable-libx264 --enable-libxvid --enable-pthreads --enable-encoder=mpeg4 --enable-encoder=aac --enable-encoder=ac3
  libavutil      52. 22.101 / 52. 22.101
  libavcodec     55.  2.100 / 55.  2.100
  libavformat    55.  0.100 / 55.  0.100
  libavdevice    55.  0.100 / 55.  0.100
  libavfilter     3. 48.105 /  3. 48.105
  libswscale      2.  2.100 /  2.  2.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Guessed Channel Layout for  Input Stream #0.1 : mono
Input #0, avi, from 'out.avi':
  Metadata:
    title           : h264 stream by H.264/MJPEG RTSP stream(ipcamera)
    encoder         : Lavf55.0.100
  Duration: 00:00:17.43, start: 0.000000, bitrate: 804 kb/s
    Stream #0:0: Video: h264 (Constrained Baseline) (H264 / 0x34363248), yuv420p, 640x480, 600 fps, 30.08 tbr, 600 tbn, 1200 tbc
    Stream #0:1: Audio: pcm_mulaw ([7][0][0][0] / 0x0007), 8000 Hz, mono, s16, 64 kb/s
File 'out.mp4' already exists. Overwrite ? [y/N] y
libfaac doesn't support this output format!
Output #0, mp4, to 'out.mp4':
  Metadata:
    title           : h264 stream by H.264/MJPEG RTSP stream(ipcamera)
    encoder         : Lavf55.0.100
    Stream #0:0: Video: h264, yuv420p, 640x480, q=2-31, 600 fps, 90k tbn, 600 tbc
    Stream #0:1: Audio: aac, 8000 Hz, mono, s16, 128 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (pcm_mulaw -> libfaac)
Error while opening encoder for output stream #0:1 - maybe incorrect parameters such as bit_rate, rate, width or height