I used an online YT to wav converter to get a wav file.
I think that SFML still has a bug. I tried this two lines of code:
buffer.loadFromFile(path.c_str());
buffer.saveToFile("test.wav");
And test.wav still is still produced like before. (the duration is 2 times longer).
I guess I leave it like this. If someone knows what could be the issue please let me know.
EDIT: if someone has a link to download a free .wav file plase let me know. But I believe that the file is not the issue here.