$> ffmpeg -i r.webm output.mp4
ffmpeg version 0.8.10-6:0.8.10-0ubuntu0.12.10.1, Copyright (c) 2000-2013 the Libav developers
  built on Feb  6 2014 20:56:10 with gcc 4.7.2
*** THIS PROGRAM IS DEPRECATED ***
This program is only provided for compatibility and will be removed in a future release. Please use avconv instead.
[matroska,webm @ 0x1d41180] Unknown entry 0xA3
    Last message repeated 129 times
[matroska,webm @ 0x1d41180] Estimating duration from bitrate, this may be inaccurate

Seems stream 0 codec frame rate differs from container frame rate: 1000.00 (1000/1) -> 30.00 (30/1)
Input #0, matroska,webm, from 'r.webm':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0.0(eng): Video: vp8, yuv420p, 640x480, PAR 1:1 DAR 4:3, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
    Stream #0.1(eng): Audio: vorbis, 16000 Hz, mono, s16 (default)
File 'output.mp4' already exists. Overwrite ? [y/N] y
[buffer @ 0x1dc64e0] w:640 h:480 pixfmt:yuv420p
Output #0, mp4, to 'output.mp4':
  Metadata:
    encoder         : Lavf53.21.1
    Stream #0.0(eng): Video: mpeg4, yuv420p, 640x480 [PAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 30 tbn, 30 tbc (default)
    Stream #0.1(eng): Audio: libvo_aacenc, 16000 Hz, mono, s16, 200 kb/s (default)
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
Press ctrl-c to stop encoding
[matroska,webm @ 0x1d41180] Unknown entry 0xA3
    Last message repeated 62 times
frame=  120 fps=  0 q=7.0 Lsize=     305kB time=1.98 bitrate=1258.2kbits/s dup=57 drop=0
video:297kB audio:5kB global headers:0kB muxing overhead 0.803083%