svoid audio_makeSampleFromSources(L<VF1<double[]>> sources, double[] pair, double gain) {
  double l = 0, r = 0;
  for i over sources: { // synchro-safe, garbage-free iteration
    VF1<double[]> source = syncGet(sources, i);
    continue if source == null;
    callF(source, pair);
    l += pair[0];
    r += pair[1];
  }
  pair[0] = l*gain; pair[1] = r*gain;
}