Add an audio track to an existing MPEG movie (using MEncoder?)

Add an audio track to an existing MPEG movie (using MEncoder?)

I have an existing MPG (MPEG-1) video file. The video has no sound. I have an audio track (currently in MP3 format, but I could record it in AC3 or some other required format if that's what it will take for it to work). The two files are the same "length" (meaning that when played, both are 5 seconds long, for example).