I took a look to my ./public/api/rhoapi-modules.js and the Rho.Mediaplayer module is there :

// Module Rho.Mediaplayer(function ($, rho, rhoUtil) {
   'use strict';
   var moduleNS = 'Rho.Mediaplayer';
   var apiReq = rhoUtil.apiReqFor(moduleNS);
   // === Mediaplayer class definition ===   function Mediaplayer() {

So, why is it still a "API module is not found" error ???

I found getJSApiModule function (in rhodes/platform/shared/api_generator/js_helpers.cpp)  that should return non null value if the module is "known" (rhodes/js_helpers.cpp at master · rhomobile/rhodes · GitHub)

It needs to use
extensions: ["mediaplayer"]

Rhomobile | Mediaplayer