By default, the Transcode API will pass through the channel configuration from the input file to the output file. If you specify a higher number of channels than the input file, silent channels will be added. If you specify fewer channels than the input file, the audio will be appropriately downmixed to match, for example a 5.1 surround mix can be downmixed to a stereo or mono output simply by specifying an output channel count of 2 or 1.