Mix audio channel with sound of video channel

Hello,

I try to add audio tracks over the video tracks to have a background sound without removing the sound of the video but I can not... The sound of my audio track remplaces the sound of the video...
 
How to do this ?
 
I use Kdenlive 0.7.2.1 on Debian Lenny.

 

If you add the video clip (that has sound) to a video track, and add the audio clip to an audio track, such that they are overlapping, they will get mixed.

Perhaps you need to adjust the volume of one of the clips; add an aduio volume effect to the relevant clip.