Onsei-nin (音声忍) (ninjutsu by voice) was a technique in which quietness of voice was used for deception. The skill includes baiboku and dakko, which is the skill of using a local dialect, alongside these two is also eavesdropping and music.
According to Sun Tzu's Art of War:
"In striking an army, in attacking a city, in killing an individual, it is necessary to know beforehand the names of the general and his attendants, his allies, his bodyguards, our spies must be instructed to discover all of these in detail."In this case a ninja could use onsei-nin to gain information.