I want to use Google Voice to transcribe an MP3 file.
How can I do this?
Can I call my Google Voice number and play the MP3? How would I do that with the best audio quality?
Yes if it is short enough you can call GVoice and play back the file
There is however a 3 minute time limit on messages ( http://productforums.google.com/forum/#!topic/voice/jZzCUQvM2BE ) so most likely you won't be able to get the result you are looking for if you want to transcribe a long file. You would have to break it up into many parts which is probably not very convenient
As far as playback goes, you need to somehow route your mp3 file into the audio in. The easiest solution would be to get an AUX cable and plug it into a smartphone (e.g., iPhone) if you have one, play the MP3 fle on that while you call into GVoice on your computer.
But the short answer is that this is probably not going to get the results you want, due to the 3 minute time limit (and, anecdotally, the longer the voice mail, the more often I get the "Message Not Transcribed" email)