My experience is that you've got to be under 50yds to hear ANY cow vocalizing other than the afore mentioned warning bark(if you heard the bark it was probably cows. I've never heard a bull bark, not to say they dont). bulls have ALOT of short whistles and short bugles that you need to be close to hear also. If they are in the timber you hear even less. Where were you at, my son was also hunting elk Oct 1st in NV.