Downmixing is turning multiple channels into fewer channels - typically 2 channels. Downmixing usually implies no change to sample rate or bit depth, so if it started with 5 channels of 24 bit/192 kHz, the result would be 2 channels of 24 bit / 192 kHz.
If your player does not allow you to specify distances to the speakers, only time delay, then use 1 foot = 1ms as that is a good approximation. The levels need to be set using an SPL meter just as you would for calibrating a receiver.