Is it that it won't play, or just Nero has taken control of that file type. You should be able to right click on the file and select another program to play it.
To change what program is associated with that file type, go to settings-control panel, folder options. Click on the File Types tab and look for the extension of the file. Highlight the extension and then click change. You should be given a list of .exe files to open it. If Media Matics isn't listed, browse for it.
Next time that file type is used, Media Matics will open.