Directly to mp3? Not possible, as far as I'm aware. What you'll need to do is either use the WAV dumper of your particular emu (if it has one) and compress to mp3 later, or if your soundcard supports it, choose the "What u hear" option from the recording input selector, open up your favourite sound editor and start recording a new file at the same time you run the emu/game you want to record.
