You were right. Reset didn't make things any better, the sounds were corrupted. I did some fiddling and found out this: looking with "View" the sound layers contained instruments but looking with "Edit->Layers" they weren't there! After editing them back, things were back to normal (or at least sounds could be heard).
The diagnostics complained about MIDI UART malfunction, I hope it's because you're supposed to connect a loopback cable for the test to work. We'll see if it works now.
Thanks for the links and hints!
Harri