There are dozens of Joomla extensions that play MP3 files but I love Simple MP3 Bar as it is indeed simple and in many cases more than adequate. Of course MP3's are great for music but why not consider adding an audio introduction or audio instructions on your site? Your audience might appreciate hearing you rather than reading a lengthy explanation. The following instructions assume you already have an MP3 file, but if you don't, check out Audacity which is a free audio editor.