Hi!
So the internal speaker has died unfortunately...
You can replace this for a new one! But it's soldered on the motherboard.
You can find it here: [guide|10670] (step 14 - next to the yellow marked IC "Sony CXM4027R MultiAV driver".
The only down side is that it's hard to find a replacement. So you should take a look at universal websites to see if they have something. Or you solder it off and ask at local repair shops if they have a replacement for you. In the worst case, you might want to buy a broken (donor) motherboard and get the speaker off from there.