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