I use Dewplayer You just download their player from the site and include it in the folder with your Hot Pot and mp3 files. Simply copy the code they provide into your Hot Pot exercise where you want it to appear - I usually choose the exercise subtitle field : see below (my mp3 file is called 01.mp3, you'd need to change that to the name of your file, of course) . I've never had any problems.
Hope this helps,
(Edited by Gordon Bateson - original submission Wednesday, 4 November 2009, 07:46 AM)