def new_note_track(env, synth):
  """
  Audio track with the frequencies.

  Parameters
  ----------
  env:
    Envelope Stream (which imposes the duration).
  synth:
    One-argument function that receives a frequency (in rad/sample) and
    returns a Stream instance (a synthesized note).

  Returns
  -------
  Endless Stream instance that joins synthesized notes.

  """
  list_env = list(env)
  return chain.from_iterable(synth(freq) * list_env for freq in freq_gen())