Ok, on a panning ssio, there are still only 2 channels out, the extra 2 sound out pins are tied to the same 2 channels. You can see that on the panning ssio schematic j3 9 and 10 are tied together, and j3 6 and 7 are tied together.
On an upright, Channels 1 and 2 are hooked up to the Left and Right speakers through the amp. On an EDOT, Channel One (both output pins) are tied to the front amp (which is tied to the Front left and right speakers), and channel 2 (both output pins) are tied to the rear amp (which is tied to the rear left and right speakers).
There is no difference between the sound eproms between upright and Enviro.