Soundblaster seems ok. Uninstalled by device manager (rebooted and) reinstalled (by OS itself).
Driver updated: 1st tried by XP itself (can't find a newer one), then downloaded by Creative and run.
Disconnected external loudspeakers: internal speaker silent.
In the property tab of the driver is written "Driver is enabled but not working properly" (but I do not think that is the cause).
Anti-Malware/Sbybot scan: system clean.
What can I do?