As you probably know, implementing speech-to-text is pretty easy with the Android API. All you have to do is just call up the API\'s intent and it will return text for you.
The API does not allow it, but see this blog post and its comments for a potential workaround. Also make sure that your file contains high quality audio (at least 16 bit and 16 kHz) to get a better transcription.
See also:
Voice recognition on android with recorded sound clip?