The channelmap. Possible values include:
      mono, stereo, surround-21, 
      quad, surround-22, surround-40,
      surround-31, surround-41,
      surround-50, surround-51,
      surround-51r, surround-70,
      surround-71 or a comma separated list of channel names:
      FL, FR, FC, LFE,
      SL, SR, FLC, FRC,
      RC, RL, RR, TC,
      TFL, TFC, TFR, TRL,
      TRC, TRR, RLC, RRC,
      FLW, FRW, LFE2, FLH,
      FCH, FRH, TFLC, TFRC,
      TSL, TSR, LLFR, RLFE,
      BC, BLC, BRC