A página sox diz:
channels CHANNELSInvoke a simple algorithm to change the number of channels in the audio signal to the given number CHANNELS: mixing if decreasing the number of channels or duplicating if increasing the number of channels.
The channels effect is invoked automatically if SoX's
-coption specifies a number of channels that is different to that of the input file(s).
Portanto, não é equivalente ( channels em vez de remix ), mas o resultado é o mesmo.