Valerio,
I can offer two suggestions:
1: Use a DSP card from NI. They run up to about 48V, last time I checked. They have circuit protection, etc.
2: If you are going to stick with the Sound Blaster, then you will need to find a hardware solution to your input signal issues. I don't know of any easy way to auto-attenuate signals to protect your sound blaster (nice pun there...blasting your sound blaster..). The only possible way to do this is with some fairly complicated hardware. Perhaps if you could elaborate on your exact application, I or someone else could offer a more appropriate suggestion.