Cheyer, a Siri co-founder and its former vice president of engineering, left recently after working in Apple's mobile software group, said one of the people, who all asked not to be identified as the matter is confidential.
The departure is at least the second among Siri's founding team since Apple bought the company in April 2010 and follows the departure of Chief Executive Officer Dag Kittlaus last year.
Apple is set to release numerous improvements to Siri with iOS 6. The company is adding support for Spanish, Italian, Korean, Mandarin and Cantonese. Siri will be optimized for use in 15 countries and provide better results for sports scores, movies, and restaurant reservations. You also can ask Siri to update your status on Facebook, post to Twitter or launch an app. Additionally, Siri gets improved hands-free functionality with a new Eyes Free mode, enabling you to interact with your iPhone using nothing more than your voice.