Okay, found the problem:
Do not install the new codec pack!
The reason is simple:
1) In my testing I thought I installed the new version. I ran the installer on my system with the "old" codec pack already installed. Since there was no errormessage from opkg I thought the task was done successfully, it was not. It was never installed!
2) After reflashing the pandora to HF5 beta 4 I had no codec pack installed and tried to install the new version. Now the lovely broken packages (like mplayer being linked against packages that are a) not part of the installation like libtinfo5 and B) in too new versions like libjpeg making the mplayer not start.
3) If you want to downgrade back to the old codec pack you have to reflash! Otherwise you won't get a working mplayer back. It is not easily possible to make opkg install the working version.
So basically: if mplayer does not start (try to start it from a terminal using the plain command 'mplayer') because of missing libs, you have to reflash and reapply the old codec pack which is available from the official website.
Sorry for the problems I caused. I fear that the only way to get a nicely updated mplayer and libs for the pandora, we need to have a build server with the specific feeds and all deps to generate a nicely tailored mplayer.