Note that with MMPM/2, a "device" can, but does not have to be a physical piece of hardware on your system. For organizational purposes, MMPM/2 creates "logical" devices as well to coordinate multimedia.
MMPM/2 defines 19 media device types altogether, but this is rather theoretical, because so far, drivers exist only for a small subset of these categories. On a typical multimedia computer, you will have the following devices:
MMPM/2 uses IOProcs and codecs to decode those audio files and send them to your sound card (see the "IOProcs" and "Codecs" page).
MMPM/2 uses IOProcs and codecs to decode video files and display them on the screen (see the "IOProcs" and "Codecs" page).
If you have the Netscape Plug-in-pack installed, you will have two such devices. The second one is then responsible for MPEG video playback. For some reason, IBM has chosen to create a second device for MPEG.