You are mixing up MCD drivers (see "MMPM2.INI") and MMIO procedures (see "MMPMMMIO.INI").
MCD drivers are for actually playing/recording audio/video content, MMIO procedures are for converting file formats (say from ADPCM to PCM) so that they become playable by MCD drivers (which only know a limited number of file formats to play, for audio this is almost exclusively: PCM) or transform to a standard format on record.
I would need to look at what commands an MCD driver needs to support but there must be something like OPEN , LOAD and also PLAY. And I seem to remember that either on OPEN , LOAD or PLAY, the MCI driver gets passed a filename or an "hmmio" which is a handle to an already opened audio/video "file" (where "file" can also mean data in memory). Search for "MCI_OPEN", "MCI_LOAD" and "MCI_PLAY" ...
Then, simply spoken, you'd just start the app with that filename (if it is a real file, otherwise you would need to create a temporary file ...). Then, in order to deal with concurrent calls, you need to deal with device IDs and possibly serializing access to that "MCD device" (maybe that replay app can only play one file at a time ...).
And then, in order to easily support multiple different replay apps, it would likely be a good idea to make that configurable via either some INI file or via some settings page in the "Multimedia Object" or some such. There are a few other functions that are mandatory to support (but won't need to do much for the envisaged use case).
And yes, the entry point of such an MCD driver (DLL) always is "mciDriverEntry".
Have a look at this, I think that's a good explanation of somebody who knows:
http://mireality.co.uk/writing/technology/os2-multimedia-chapter/media-control-interface/By the way: MCD drivers are added via a CARDINFO.DLL resource DLL. It's some work to write one but it's not extremely difficult (maybe there also exists some script approach but I don't know). An example has been the "Timidity MCD" which essentially wrapped the Timidity SW so that you could play MIDI files with a double click.