So, the correct plural form of medium is media. Of course, there are exceptions, and the words have taken on new usages (such as adding a definitive article "the" to media, making it singular), but isn't the word multimedia redundant? What you are really trying to say is multiple mediums -- or just media.
I imagine it started in the 50s or 60s. Did it start as a "buzzword" or some sort of industry jargon? Perhaps it is a byproduct of the aforementioned exceptions, in that "the media" became singular, and thus a new pluralized form had to be developed?