A coworker and I once tried a little experiment to find out about the sound vented broadhead make. We went out to the range and took turns standing downrange (behind a very large enclosure) and had the other guy shoot broadhead tipped arrows past. We used a few different brands of heads, some with a very noticeable whistle, some with no sound. To the archer shooting you could hear the hissing arrows as soon as it left the bow.
Interestingly, to the person standing behind the enclosure you could hear the bow shoot but you couldn't hear the whistle until the arrow was past.
The reason we tried this was someone with a whole lot more experience related this to us. I thought it was bogus until I heard it. So, I no longer care if the arrow hisses downrange. I figure if the animal hears it the arrow came out of the ribcage and it's too late.
CS