Why does Zero have such a strong reaction to Shizuka Hio's voice?
I guess it was what you said, a bond that was made due to Shizuka biting and turning Zero into a vampire.

During one episode (I'm not sure what episode), Zero has figured out that she must be Shizuka. Maria taunts Zero to pull the trigger, but he discovers he is unable to due to the bond between them. And it is said that once a human was bitten by a vampire, that vampire becomes his/her master and that human can be controlled by his/her master (like when Shizuka froze him when she was about to drink Yuki's blood).