For a long time I couldn't figure out why I couldn't use ffmpeg to convert my videos from my sony handycam. When I open it with Windows Movie Maker it messes up the aspect ratio, so I didn't want to do that, either. More recently my mom had trouble playing the files because she doesn't have the mpeg-2 codec installed. I went digging through the internet for some sort of software video converter, but, finally, I decided to give ffmpeg another try. After reading the error message more carefully(ah duh), the problem was not the aspect ratio or that it's mpeg-2, but audio related. I guess mpeg-1 doesn't support 5:1 surround sound, so just forcing it to 2 audio channels did the trick. Man, I feel genuinely stupid. The command I ended up using(or similar) was:
ffmpeg -i emma\ roll\ over.mpg -b 900 -s 360x240 -ac 2 emma_roll_over_web2.mpg
Update: you might want to make the bitrate(-b) higher, maybe 900k instead of 900 ;)