I don't quite know what you mean by "surround sound software." I can only assume you mean software that uses surround sound.
First of all, for surround sound software to be any good, I would imagine you'd first need some surround sound
hardware.
The G5 towers have an optical S/PDIF out, and you can buy cards (PCI for towers, or FireWire/USB ones for 'Books) that have analogue surround sound and/or S/PDIF digital outs on them.
M-Audio is a popular maker of such devices.
As for software that actually uses >2 channel sound, I think the Panther DVD player does... maybe a game or two, but not very many support it, if any do at all. AFAIK, there's no standard/easy way to do it on Mac, so support is somewhat limited.
I hope that answers you question...