Do you want the file to play on the site or be available for download? Or both?
In general, MP3 is the most compatible with all computer operating systems, but you could also sent up a QuickTime streaming file that would play automatically when visitors to the site hit the page on which the player is located. From there, they could download, unless you want to block that.
Please check out one of my sites, http://theparacast.com for an example of how we set this up.
Co-Host, The Paracast
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