I have listened to the video several times, and I stand by my perception that only one person audibly yells "boo."

I hear a loud "boo," then a difficult to decipher noise — which could be an ugh response to the booer — and then a little more of a boo, which sounds like the original guy.

ADDED: From Jim Geraghty (who faults Rick Santorum for failing to thank the soldier for his service):

Sarah Rumpf, who was in attendance, writes:

... There was audible booing after his question . . . however, please note that it was not the crowd booing. It was only one or two people.

I was at the debate, in the audience on the right hand side about halfway back...  The person who booed was just a few rows in front of us. The booing got an immediate and angry reaction from nearly everyone sitting around him, who hissed and shushed at him. Lots of loud gasps, “Shhhh!” “No!” “Shut up, you idiot!” etc.

See, that's my point. The sound after the loud boo was not more booing or any negativity directed at the soldier. It was hostility to the booer.

Also, Santorum says:  "I did not hear those boos . . . If I had, I would have said, 'Don’t do that, that man is serving his country and we ought to thank him for his service.'" As I said in my original post on the subject, these inferences about how the candidates responded to the audience "assumes they hear the sounds and can immediately correctly interpret whether it's an approval or disapproval sound and what it refers to."

In fact, if Rumpf is correct, most of the people watching at home were failing to correctly interpret the sounds they heard. If only one guy booed, and all the other voices were people trying to shush him, and the candidate had launched into some righteous chastising of the the audience, it would have opened that candidate to criticism about how he assumed the people were intolerant and ungrateful.