Ubuntu Multimedia :: Extracting Audio From MP4 Video File

I'm trying to extract the audio from an mp4 video file and convert it to mp3 using ffmpeg. The problem is that the resulting audio file is only 3m2s long, whereas the video file is 8m15s long. Anyone know why the audio file is being truncated? Am I doing something wrong? Apart from being truncated the mp3 file plays perfectly. Here's what ffmpeg has to say about the video file:

Code:
Seems stream 1 codec frame rate differs from container frame rate: 59.83 (59834/1000) -> 29.92 (359/12)
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Part1.mp4':
Duration: 00:08:15.60, start: 0.000000, bitrate: 541 kb/s
Stream #0.0(und): Audio: aac, 44100 Hz, stereo, s16
Stream #0.1(und): Video: h264, yuv420p, 1152x720, 29.92 tbr, 29.92 tbn, 59.83 tbc

And here's the command I'm using to extract and convert the audio:
Code:
ffmpeg -i Part1.mp4 -ab 128k Part1.mp3