I never saw BeOS do that with video, but I heard it do it with MP3 files. SoundPlay was a kind of crazy bananas MP3 player -- it could basically act as a mixer,

I never saw BeOS do that with video, but I heard it do it with MP3 files. SoundPlay was a kind of crazy bananas MP3 player -- it could basically act as a mixer, letting you not only "queue up" multiple files but play each of them simultaneously at different volume levels and even different speeds. I've still never seen anything like it outside of DJ software.