Found an answer that works without modifying the file. If you change the rate
of the track you are editing, it slows the whole track. e.g. if you set of a
44.1kHz track to 11.025kHz it plays at 1/4 speed. When finished, just set the
sample rate back to the original.
Setting sample rate is done by double clicking on the track title, and using the
"set rate" part of the menu that appears. I found this accidentally while
trying to change a track from 44.1 to 16kHZ so it can be a ringtone. BTW I now
know what U2 will sound like at 80!