I have now tested the headphone jack and there is perfect sound in the headphones!
So this tells me that it could be the soldering related to the audio/internal speakers. Can anyone guide me to how I locate the relevant solderings? I haven't got so much an idea of where the audio board actually is, so a photo or something similar could be very cool! I know how to use a soldering iron so it is a matter of finding out where to use it :-)
