Warhol wrote:I read everywhere I had to go here: /boot/config.txt; to change something, but I don't understand how I should do .. where do I enter this command?
Depending on what HDMI mode is being used, uncommenting hdmi_drive=2 in /boot/config.txt will solve your problem.
At the terminal prompt, type: sudo nano /boot/config.txt
Then remove the pound sign in front of hdmi_drive=2. Press Ctrl+X in order to quit the nano text editor. (Make sure to save your changes!) Then reboot the Pi and try launching a game (or play a sound file) to see if sound over HDMI now works.