Microsoft is buying AI speech tech firm Nuance for $19.7 billion, bolstering the Redmond, Washington-based tech giant's prowess in voice recognition and giving it further leverage in the health care market, where Nuance sells many products. Microsoft will pay $56 per share for Nuance, a 23 percent premium over the company's closing price last Friday. The deal includes Nuance's net debt. Nuance is best known for its Dragon software, which uses deep learning to transcribe speech and improves its accuracy over time by adapting to a user's voice. Nuance has licensed this tech for many services and applications, including, most famously, Apple's digital assistant Siri.