Okay I see the problem of be-at.tv now:
The videostream is missing frames every now and then. So if I use a constant frame rate, the video will always be shorter than the audiostream. I figured it out when I saw, that sometimes my downloaded video is getting slower and faster.
When watching the video via the website, the video get's resynced to the audio every few seconds.
So, right now it's turning out into the pro-question
: How can I merge the video-chunks AND have it synced to the audio?