This is already in Windows 10 Mobile in the current preview build (10586.29), in supported keyboard languages (such as English (U.S.) and English (U.K.)).
With the keyboard active, you will see a microphone icon just above the keyboard. Tapping on that will prompt your phone to listen to your voice - whatever you say audibly will be transcribed into text, with accuracy of transcription varying greatly depending on the accuracy of your pronunciation, diction and ambient noise.
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12-10-2015 10:22 AM