I prefer to charge per audio minute - it means the client will know up front exactly what the cost will be, plus it means I can make more money than an hourly rate.

I recently did a 32 minute audio file and it took me one hour (no I dont type at 200 wpm, it was very clear and a fairly slow speaker). With this client I would have been paid $25 an hour so effectively that's all I'd have got, but by charging per audio minute I got over $39.

It doesnt work with all clients as one of mine do audio plus they might have written notes, I basically charge them an hourly rate BUT this is one of my first clients and I'm happy to keep it that way. If I had a new client approach me that did similar I'd charge them per audio minute for the audio and then an hour rate for the written content.