It is important to clarify a common mistake. When people see a file with a .AVI extension, they immediately conclude that it is not an MPEG file. That is not true. At least not entirely. Contrary to popular belief such a file can contain MPEG-1 video.
You see, a codec is not the same as a container format. Examples of video codecs are: MPEG-1, MPEG-2, MPEG-4 (DivX), Indeo5, 3ivx. Examples of container formats are: MPG, AVI, ASF.
In theory, you can put an OpenDivX video and MP3 audio into an MPG container. However, most players will not play it, since they expect MPEG-1 video and MP2 audio (unlike AVI, MPG does not have the necessary fields to describe its video and audio streams). Or you might put MPEG-1 video into an AVI file. FFmpeg and MEncoder can create these files.