I use Audacity to edit my podcasts, but I would not suggest it unless you are willing to spend several hours figuring out how to use it. (its not really hard, but it does take a little getting used to).
I would suggest that VLC would be a good choice. If you look in the “Playback” menu there is a playback speed. It can go from 0.25x to 4.00x. It works for both audio and video.
Warning if you play audio slowly it sounds a little funny.