$ ogmcat file1.ogg file2.ogg file3.ogg -o merged.ogg
$ avimerge -i file1.avi file2.avi file3.avi -o merged.avi
mpegs are even nicer, you can simply concatenate them together
$ cat file1.mpg file2.mpg file3.mpg > merged.mpg
$ mkvmerge -o MyMovie-with-sound.mkv -A MyMovie.avi MyMovie.ogg
