Character, indicating whether the “left” or “right” channel should be extracted,
or whether “both” channels should be averaged.
left
Object of class Wave containing monophonic sound,
to be used for the left channel.
right
Object of class Wave containing monophonic sound,
to be used for the right channel (if missing, the left channel is duplicated).
If right is missing, stereo returns whether left is
stereo (TRUE) or mono (FALSE).