I haven't been able to open the link, so I'll remain skeptical...
Whistles are a categorie of audible signal that I can't remember being useful for any of the searches I've been on, but that is not quite a scientific study. I can remember three times when I located the person by shouting: once on a clear day over a mountainside, once in a snowstorm in a wooded/rocky area at daybreak and once in the middle of the night... yelling every so often..."Jeeeeessssssseeeeeeeeeeeee...Jeeeeessssssseeeeeeeeeeeee"...the teen was sleeping soundly in the heavy bush about 3 feet off the trail and jumped straight up when he heard my yell...scared the crap out of me...
Oh, one more came to mind. Middle of the night in farmland, two youngsters went out to play after school and never returned. Introduced myself over the loudspeaker and assured them they weren't in trouble (youngsters will hide from adults when lost, believing they will catch hell when found). I told them their parents were worried and told them they needed to help us find them by making noise, and instructed them to "make noise...Now!". They started to yell, which sounded a lot like a herd of sheep in the night... the firemen who had been combing the area were now able to find them... the only part visible was their faces... they had tried to cross an "empty" irrigation canal and had sunk in the mud to the point where less than half of their head remained clear... The mud is a tremendous force and it took several adults several minutes, probably close to half an hour, to keep the kids from sinking more and to free them from the muck.
If the parents had waited another hour to call, probably would have never known what had happened to them.
Gunshots (into a safe target) travel very well, but echo so much it creates pin-pointing problems at a distance.