You can edit the file in Audacity. (speeding up stuff is fast and sounds
good - I do it all the time for pacing myself when jogging. slowing stuff
down is time-consuming and usually sounds bad.)
If you want to do it in real time without altering the original files there
should be plenty of plugins for a.o. WinAmp to do this. What you are looking
for i something that can do time stretching and/or pitch shifting. http://www.winamp.com/plugins/details.php?id=138562 should be capable, for
instance.