String [] command = new String []{
                "-i", mVoiceFile.getAbsolutePath(),
                "-i", mBeatFile.getAbsolutePath(),
                "-filter_complex",
                "[0:a]volume=3.0[a1];[1:a]volume=1.0[a2];[a1][a2]amerge=inputs=2,pan=stereo|c0<c0+c2|c1<c1+c3[aout]",
                "-map",
                "[aout]",
                "-ac",
                "-2",
                mRapFile.getAbsolutePath()};