It is sometimes hard to find the tick that is messing things up. So, you start from the easiest. First, bring up the Volume Control (Start-Programs-Accessories-Entertainment-Volume Control) and make sure Mute is not checked, and all the volumes are turned up. Second, make sure that the external amplifier is working. Third, (and you probably already tried the first two) go into your System Manager Hardware tab and make sure that there is no exclamation point next to your Sound and that it recognizes the proper drivers.

System manager? Did you mean device manager or i don't know what you're talking about...
 
EDIT: I somehow believe that the driver is f***d up, because i get some crashes with a blue screen and it mentions sth about drivers being bad or sth like that...

Yes, your device manager. If it is your drivers, you can download the most recent from here: http://www.majorgeeks.com/Realtek_AC97_Drivers_d4400.html
 
You can go into your device manager, and uninstall the sound device. Windows will want to install a new one. You can then run the executible from Windows Explorer that you downloaded earlier and it should provide the drivers for your interface.

It wasn't about the driver afterall...
 
There is a jumper on the mb and it wasn't set correctly