The Google Drive (Docs) idea is a pretty good one if you have
Soundflower installed.
You just change both your Audio Input and Audio Output to "Soundflower (2ch)", create a new Google Docs (word processing) document and select
Tools>Voice Typing, click the microphone (you'll have to give permission the first time), and then start playing your audio file. Since the audio isn't being routed to your speakers you won't hear it. You'll see the transcription in the document as the audio file plays.