MPEG-1 videos might be created with only audio track present and no video, albeit the purpose of such conversion would be pretty limited. This means that mp3 to mpg conversion is doable as well and the result will be MPEG container with just audio track present.
Open MP3 file Open MPG fileA multimedia conversion software
A multimedia conversion utility
A multimedia converter
A multifunctional media converter for Windows
Universal video converting software for Mac
Universal All Purpose Free Media, Audio & Video Converter Player Recorder
Video converting software