There are no file associations as such ... the system isn't actually using the paradigm of files and media player applications at all. Each piece of media that is active, is abstracted through a "media stream" object. The players are actually DCE "devices" that must integrate with the DCE Router protocols and accept stream objects for play back. So any "association" that exists, such as it is, happens within the Media Plugin when it is creating the stream and sending the MH Play message to a playback DCE device... and this is hardcoded... so you will have to grab the source code, make the modifications you want to change the "association" and recompile it, then replace the Media Plugin with your code.
Better yet, you could pull out the hardcoding into some Device Data entries and provide the association mapping you are looking for, as that could potentially be useful for others.