Why WAV transcribes so well
WAV is uncompressed, so no detail of the voice is lost. Recordings from devices like the Zoom H1n, Tascam or a USB microphone usually produce the most accurate transcripts.
Large files
WAV files are big, about 10 MB per minute in CD quality. If yours is over the upload limit, convert it to MP3 or M4A first; transcription quality stays almost the same.