We've got a show that's to be delivered in 5.1 and I'm assuming that the client will want to hear it that way when we online it and do the layback. 

I'm currently feeding the client monitor (KDL-46XBR4) thru HDMI taking the speaker outs to a 12x2 mixing board then to our stereo Yamaha's.

What's the correct way to do this and what's the cheaper alternative you have found?

Thanks,
Larry 

2x 3 GHz Quad MacPro 

8GB Ram

Symphony Nitris DX 3.5

BVM L230

The Nitris DX does have the ability to output 5.1, but if you only have 2 speakers then there's not a lot you can do.  If the sound mix has already been done, then get the post house to supply you with the stereo mix.  (there's always a stereo version of a 5.1 mix- or at the least a fold-down).

If you're worried about sound, remind your client (in the nicest possible way) that you deal solely with pictures, and any sound played in your suite is just a guide.  Take no responsibility for sound during your online, except to check sync.

If you want to set up your room for 5.1 you can do this for $200 or $20,000.  We have a presentation room set up with a domestic Sony A/V amp.  But a nice as can sound, it sounds nothing like the dubbing studio where your client has probably just come from, so be careful if you decide to go down this route.