Lisa, I think you mean a CO (carbon monoxide) gas detector. CO2 is relatively harmless (except for the whole Green House Effect thing). We breath out the stuff after all.
Carbon Monoxide on the other hand can kill you. Even if it doesn't kill you, it can quickly cause brain damage in high enough concentrations. It is odorless, colorless, and hard to detect without help from devices like these.
Grimp... why would you want it to be silent?? Isn't the point to be woken up if something might otherwise kill you??