Some audio clips don't sound when I export a Premiere Pro project

Hello!

I'm using the version CS6 (in both Premiere and Media Encoder)

Sometimes I put a voice clip and a music file "one above the other" in a Premiere project, so that they sound at the same time (something like this, don't pay attention to the language haha):

...But my problem is that Media Encoder sometimes makes both of them sound, and sometimes it makes only the voice sound; in the final exported video.

I literally just exported the video twice, and the first time the music didn't sound but the second time it did...

I have no idea what's going on, am I doing something wrong?

Hope you can help me out because this really is a problem, since exporting the video takes quite a long time.

Thanks!!