Okay here is a thread in the german forum:
Flash Tutorial
I think you can follow the steps and the screenshoots without German
Here is the translation for the part where we add the sound:
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Now we will add the music. To do that we click File -> Import -> Import to Stage and select the mp3 file.
To play the music click on the timeline and on Layer 1 to select the layer.
The menu on the right should look like this:
Choose the mp3 file at the "Name" drop box in the sound section.
At Sync you have to choose "Start" and in the 2nd field you have to choose "Loop".
Done! To test it, use: Control -> Test Movie.
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original from "eins"
The upload page is currently down.
Usually it is this one:
http://z0r.de/up/