Well, I'm not particularly proud of it, but I managed to fix the internal sound by placing a precisely shaped piece of aluminium foil over the headphone port.
At first, I thought a pseudo-button needed to be pressed, but the problem was solved by short-circuiting two contacts inside the jack. It took me a while to create the exact shape of the foil to short-circuit just the pins I needed, but I finally did it. I don't think I'll touch it while it works. Now, I'm going to do some retro gaming!
Jan "Jenkings" Škoda - blog
Simplifying IT, Amplifying Knowledge