I have noticed that many subordinate toms will gobble quiter than the boss. This is because they have been whipped and don't want another beating. Hens yelp at different volumes, and gobblers sound off at different ones as well. Other reasons for the gobbler being quiter can be terrain or just which way the gobbler is facing. If they were walking towards your friends and away from you...the gobble would be louder to them and vise versa. I am no biologist but this is my opinion. I just don't think a gobbler has no control of his voice so to speak...he gobbles as loud or as soft as he wants.