Convert an MP3 from 48000 to 44100 Hz

ffmpeg doesn't look to be the appropriate tool; I'd normally use sox for audio-only files.

$ sox file1.mpg -r 44100 file1-enc.mpg

If you want to try using ffmpeg anyway, I think the correct command would be

$ ffmpeg -i file1.mpg -ar 44100 file1-enc.mpg

Convert bit rate

$ sox file1.mpg -b 16 file1-enc.mpg