What do cloud-based speech-to-text services upload to the cloud?
Its probably safe to assume that all of the major companies send enough information to reconstruct the audio. This is because having that audio for training is such a valuable resource. A certain percentage of the audio segments will be listened to and transcribed by a human annotator. 

Also features in these systems are more complicated than MFCCs. You obviously want to offload computing to the cloud so that's another reason to expect that they send the whole audio.