Hi...
Some news, the construction wasn't very effective, the contacts often didn't close when I played with the function channels. In addition, the cabeling sometimes broke and I had zu renew the soldered contacts.
So I decided to search for an alternative solution and I found a shield from Adafruit, the VS1053 MP3-Codec shield. It was exactly what I was searching for.
Here's a link to the product
Adafruit VS1053.
In addition there's a great
tutorial for the shield.
The board has changed a bit.
The control is the same, but the "Previous" function is removed, it wasn't needed.
Now that it's working I'll place it into the Black Devil.