The Stream instruction are for playing files over the windows media player filter.
As many movie, sound or music files as supported by the hardware can be played simultaneously - for instance, to play two midi files at the same time you'd normally need two sound cards. All files playable with the windows media player can be used with this instruction - .avi, .mpg, .wmv, .wma, .wav, .mid, .mp3, .ogg and so on, even some still image formats like .bmp or .jpg..