Ok, not much luck so far.
Replaced the wlan component, still the same problem (sound only when recovering from standby).
Nand backup done (1 bad block : 3860 if I recall correctly).
Format lflash done
5.00M33-6 reflashed.
Restarted the PSP and the blue screen (press O to restore default settings in multiple languages) came up. Pressed O and the screen faded away (just like the screen went bad) and after a few seconds the PSP turned off.
Turned it on again and the blue screen was there again. This goes on and on
Restored the NAND and we're back to square one.
I'm open to suggestions...