Well, we knew this already:
Why Men Don't Listen to Women
Bottom line: Men have to work harder deciphering what women are saying because they use the auditory part of the brain that processes music, not human voices. Men's brains are not designed to listen to women's voices. It's not the pitch of the woman's voice, but rather the vibration and number of sound waves that cause the problem, notes Discovery News.
[May I point out that pitch is directly related to frequency, which is number of waves per given time interval. Duh.]
But guys have no trouble at all hearing each other because men use a much simpler brain mechanism at the back of the brain to decipher another man's voice and recognize it as speech.
On a related note, check this out:
How to tell the sex of a bird