Okay... After some more research, I have a better response for you.
As you may already know, support for loop is dependent upon the individual player that we're talking about (i.e., Quicktime, Windows Media Player, RealPlayer, etc.). Each player not only will support this feature differently, but each version of each player could potentially treat or ignore this feature differently. This is something that's outside of our control though, as it's completely dependent upon the software that your website visitor has installed on their computer.
In the case of Windows Media Player, if you are running the latest version of the player on your computer (v11 right now), then looping is not supported
by that specific player. Earlier versions of WMP mostly do though.
With RealPlayer, looping appears to be supported, as I can't find anything that says otherwise. And I have confirmed that the looping mark-up is correctly added to the Media Module, per the RealPlayer documentation
, as long as the player version is G2 or newer.
Lastly, the documentation for Quicktime says that looping is supported
, with a couple of exceptions.