question: public String combineMix ( List < String > files , String outFile ) { ArrayList < String > cmd = new ArrayList < String > ( ) ; cmd . add ( soxBin ) ; cmd . add ( "-m" ) ; for ( String file : files ) { cmd . add ( "-v" ) ; cmd . add ( "1.0" ) ; cmd . add ( file ) ; } cmd . add ( outFile ) ; try { int rc = execSox ( cmd , callback ) ; if ( rc != 0 ) { outFile = null ; } } catch ( IOException e ) { e . printStackTrace ( ) ; } catch ( InterruptedException e ) { e . printStackTrace ( ) ; } return outFile ; }
target:  Combine and mix audio files sox -m -v 1.0 file[0] -v 1.0 file[1] ... -v 1.0 file[n] outFile TODO support passing of volume