Thomas:

The way this currently works is that the module will inject preferential players in the mark-up.  If the browser has that player, it uses it.  If it doesn't, it uses whatever player that is installed on the client machine that will play the associated file type.  The mark-up couldn't be created in any other way and still support multiple use cases.

As an example...

If you upload an MP4 and have only Windows Media Player installed, the Media Module will:

1. Media Module loads markup for Quicktime in the web page
2. Your computer will use WMP to play the media because Quicktime is not installed

If you do the same thing with Quicktime also installed, you will have a slightly different experience.

1. Media Module loads markup for Quicktime
2. Your web browser ignores the player preference in your system
3. Your media is played using Quicktime

This is currently by-design due to making sure that the module can span multiple use cases.  Being able to play local media across multiple browsers and system configurations is a surprisingly complicated - very complicated - thing to plan for and address.

@Nikerym:

You have also stumped me... If the environments are clones and IIS configurations the same (assuming the same version of .Net, IIS, etc.), then you should have the same experience on both sites.

There must be something different.  The markup generated by the module is only conditional by the file extension at this time.  Nothing else.  :(