"Bronchophony literaly means "bronchus sound"
While auscultating remote from the bronchi and larynx
the examiner can hear the speaking patient's laryngeal /bronchial sounds
while not being able to distinguish the words.
The choice of "ninety-nine" is the unfortunate result of a literal translation.
The test was originally described by a German physician who used the phrase "neun und neunzig"
which he found would cause maximum vibration of the chest.
The translation
"ninety-nine"
has fewer vowels and is less effective in evoking the phenomenon.
Better phrases in English include ""Scooby Doo"