Can you provide this mp3 file ?

Created attachment 34383 [details]
MP3 file in which the bitrate is not detected

A sample MP3 has been uploaded.

that works fine here. What version of rhythmbox/gstreamer/gst-plugins do you
use? do you still have this issue?

I have the same problem with the lastest Rhythmbox and gstreamer version.
ii  gstreamer0.10-alsa                    0.10.2-1ubuntu2
ii  gstreamer0.10-ffmpeg                  0.10.0-0ubuntu2
ii  gstreamer0.10-gnomevfs                0.10.2-1ubuntu2  
ii  gstreamer0.10-plugins-base            0.10.2-1ubuntu2
ii  gstreamer0.10-plugins-good            0.10.1-1ubuntu1
ii  gstreamer0.10-x                       0.10.2-1ubuntu2    
ii  libgstreamer0.10-0                    0.10.2-1ubuntu1 
ii  rhythmbox                             0.9.2cvs20060124-0ubuntu1

Currently we just report whatever gstreamer gives us, and for some variable-bitrate files that is "0" for Unknown. I guess if it is 0, we could approximate the bitrate with filesize/duration.

I opened bug 302625 to fix mp3 bitrate reporting in gstreamer 0.8, but the changes made there haven't been ported to gstreamer 0.10.  I've now opened bug 329575 for that and various other bugfixes to the gstreamer mad element from gstreamer 0.8.

With current versions of rhythmbox and gstreamer, this is working about as well as can be expected.