code: public final static Sample mix(Sample sample1, double volAdj1,
                                     Sample sample2, double volAdj2) {
          double[] s1_ary = sample1.getInterleavedSamples();
          double[] s2_ary = sample2.getInterleavedSamples();
  
          //
          double[] mixed = mix(s1_ary, volAdj1, s2_ary, volAdj2);
  
          return buildSample(sample1.getSampleCount(), mixed);
      }
docs: <p>mix.</p>
  
  @param sample1 a {@link ameba.captcha.audio.Sample} object.
  @param volAdj1 a double.
  @param sample2 a {@link ameba.captcha.audio.Sample} object.
  @param volAdj2 a double.
  @return a {@link ameba.captcha.audio.Sample} object.