libav-tools package.
Convert ogg into mpg:
avconv -i input.ogv -f 24 output.mpg
You must specify frame rate, as the following fails because MPEG does not support the default 15fps:
avconv -i input.ogv output.mpg
Cut music file starting from second 1 to second 5:
avconv -i input.mp3 -ss 1 -t 5 output.mp3
Times may be either in seconds or in hh:mm:ss[.xxx] form.
Overwrite output file without asking:
-y
Concatenate files: http://stackoverflow.com/questions/18552901/how-to-merge-videos-by-avconv