Which animal's voice has the lowest frequency and which one's has the highest?

The highest frequency animals are whales and sun bats.

The answer is at the very bottom:
http://www.thenakedscientists.com/HTML/content/interviews/interview/607/

The lowest might be that of blue whales or elephants, both of which emit low-frequency (sub-sonic) sounds on occasion to identify themselves or to communicate.

The human voice generally does not descend below 70 VPS, but some Russian bass singers have a technique to sing as low as 30 VPS. The Human ear can't usually differenciate tones under 40 VPS or even hear them under 20 VPS.