With elementary streams, there are separate files for the video and audio components.
Elementary streams are required by some applications.
Compressor automatically creates an elementary stream when only the Video or Audio
tab is enabled, but not both. To create elementary streams of both the video and the
audio, you must use two settings—one with the Video tab enabled using the file extension
of .m1v and a second with the Audio tab enabled using the file extension of .m1a.