I used to do offline mixes with a (partly generated) shell script that calls several sox instances to do the mixing/tempo change/volume adjustments. As a CLI person, I liked that approach, although I would be happy for something a little more streamlined. I wonder if anyone knows an offline mixing tool in the spirit of KISS, without a GUI? I guess Csound would be a better backend then sox, but I lack the fluency in Csound to do that...