More than 10 years ago, Apple added MPEG playback to QuickTime. In order to work around QuickTime not having support for bi-directional difference frames, and some issues with MPEG's format being a bitstream rather than a byte stream, this was done my making it a special media type rather than showing up as separate video and audio tracks. As a side effect of this, exporting audio from MPEG movies didn't work. This was one of those features that was annoying to few enough people and just enough work that it has been continually deferred from release to release (hey, I spent 6 years there and never got to it either). There have been MPEG stream manipulation tools from Sparkle onwards, but they have always needed a deep understanding of the format, a degree in computer science and lots of patience to use.
Now that there are video cameras and digital cameras that record MPEG directly, this is suddenly a lot more annoying, but fortunately there is an answer:
DropDV: Convert MPEG to DV: DropDV is a Mac OS X droplet which converts MPEG video into DV video streams. This allows the video to be edited in iMovie, Premiere, Final Cut, or any other DV video editing system.
Handles both video and audio
Uses high quality bicubic scaling for the best video image
Decodes in YUV color space, other tools use RGB.
Supports both NTSC and PAL output
A Simple, drag-and-drop interface