FWIW, I have been experimenting with Dictation in Mountain Lion the last few hours, and when using a USB headset microphone in a quiet room, I have been very impressed with its accuracy. However this is again in short bursts of say a paragraph length then pausing to send the data to Apple for conversion to text and waiting for it to come back before proceeding. It appears the limitations I mentioned earlier in relation to my son's medical dictation still hold true here, but the cost — FREE — greatly increases its viability for casual use. I doubt seriously it would be satisfactory in the environment you are wanting.
Out of curiosity, I checked with the new Information Technology director at our church to see what process they use for transcribing the Sunday Sermons and she indicated that after extensive testing both here and at her previous church they still use manual transcriptions. She said the best system they tested — in a situation that appears similar to yours — costs well into five figures and still required extensive manual correction and editing. Even using very expensive commercial grade wireless headphones, if there was background noise such as the organ, or the congregation moving around, the translation accuracy approached zero.
