I am in search of audio editing software that can smoothly "overlay" a
sound file over another (MP3 preferably), in a batch process.

I want to create MP3 music files with a 10 second clip about 30
seconds in, dubbed over the music, which has faded to the background
and then fades back to full after the clip. I would also accept a
program that will "insert" another sound file and fade out and back
into the affected file, but that will loose the beat so I'd prefer to
find another.

The key is "batch process."

If anyone knows of an editor with -advanced- command line funtion that
could alow this to be done manually, and fully, from the command line
- that will be perfect (even better actually.)

Thanks!