I guess I already figured it out.
Because I create the sf:Sound and sf::SoundBuffer within the function, these are local variables and are removed as soon as the code has been executed. Even while play() will play within a separate thread, this code will reference to a SoundBuffer that is not accessible, am I right?
So for my code to work I just have to make the sf::Sound(Buffer) objects part of the class that uses them.
Sorry for my post, guess I should've been more patient.
Greetings,
Rob