It's possible in the same way as with a Windows PC, you need a 5.1 kit, and some cable, and a USB DAC if you want it to not sound crappy. Personally I have my imac plugged into a lovely pair of Floorstanding speakers and it's never sounded better
I had a cakewalk UA-1G USB audio interface. I hooked it up to my iMac and now have SPDIF output, guitar Input and lots of other goodies and most importantly, being a SPDIF output, I have volume control on the mac !!!!
My Studio Monitor speakers sound just great with this setup.