audio muted after rendering

I am new to ubuntu (v11.10 )and recently installed kdenlive Version using software center. I am trying to add an audio track (mp3 file ) to a video footage (mp4 file )which I have taken.I see the audio being played in the project monitor but when I rendre the project the final ouput file(mp4) doesn't have the audio at all, its muted. There are no warnings or error reports generated.

is there something special that need to be done to add an audio track or is this some bug in the software ?

thanks for the help.