A muxed MPEG-1 file is a MPEG-1 file that has the video and audio tracks mixed together. You can run into problems encoding these types of files using some compression applications. So it's a good idea to demux the MPEG-1 file. Once you compress the video you can re-assemble the audio and video in Quicktime Pro. This will give you a seperate audio file. If you don't have QuickTime Pro available then you can assemble your files in Final Cut Express or Final Cut Pro then export to QuickTime. It's not the best work flow but it will work.
To demux your MPEG-1 file open your file in MPEG Streamclip and navigate to Demux to AIFF via the File pull-down menu. You don't have to bother with Demuxing the video because when you compress your MPEG-1 muxed file in many cases you will get a video only file.
Download MPEG Streamclip for Mac or Windows XP
