I used to use a digital recorder that recorded directly to mp3 or wma (windows media). The one I specifically used was by Pogo (search for it on Amazon), although there were others that do the same job. These are flash memory based players by the way.
HOWEVER, I think it's a waste of time to go through audio... usually lectures take longer than they need to, plus it's hard to find the specific timepoint that you're looking for. I think it's a better use of time to either look something up in a book or to ask one of your friendly classmates.