ffmpeg: setting audio bitrate

Hi, I have the lastest ffmpeg medibuntu installed and I have problem encoding video with audio bitrate different than default.

For e.g., if do

ffmpeg -i in.avi -title "A title" -vcodec copy -acodec aac -ac 2 -ar 48000 -ab 128 out.avi

ffmpeg produces a video with no sound. The output is following
===============================================
Seems stream 0 codec frame rate differs from container frame rate: 30000.00 (30000/1) -> 29.97 (30000/1001)
Input #0, avi, from 'in.avi':
  Duration: 00:22:04.6, start: 0.000000, bitrate: 1699 kb/s
  Stream #0.0: Video: mpeg4, yuv420p, 640x480, 29.97 fps(r)
  Stream #0.1: Audio: ac3, 48000 Hz, stereo, 192 kb/s
Output #0, avi, to 'out.avi':
  Stream #0.0: Video: mpeg4, yuv420p, 640x480, q=2-31, 29.97 fps(c)
  Stream #0.1: Audio: aac, 48000 Hz, stereo, 0 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
===============================================

The same thing (nosound) happens if I set "-ab 128". However if I omit -ab option, ffmpeg produces video with sound but with the default 64kbps bitrate. Replacing "-acodec acc" to "-acodec mp3" doesn't help.

Is this a bug? or did I do something wrong ?