Audio mixing doesn't work in certain project

I've been working on a project for a couple of weeks, and I finally got to the point of adding background music. The problem is that it doesn't work. The music only plays in parts of the project where there are no video, but only a title clip. As soon as it gets to the point where an actual video clip (with speech) starts, the music is muted. I want both audio tracks to be mixed together. How can I fix this? It has worked in my previous projects, and I even tried creating a new project with the same audio clip, and there mixing works just fine. I don't want to start from scratch with this project... I'm using Kdenlive 0.8.3 on Linux Mint 12.

Could someone please help? I actually did start from scratch, created a new project file and worked on it for a few hours. The audio of the music and video tracks mixed together fine. But now that I opened Kdenlive and reloaded the new project file, it's again not working. This is driving me crazy!

0.8.3. sounds like a development version of Kdenlive, as the official release is

Maybe you have got a somewhat bad build. Did you try to switch to or another build from the development branch?