What do you consider 'cheap...' - this means different things to different people...

Kevin Klimek:

I know that surround can be output via HDMI, which works well with the Nitris DX.  Other 3rd party hardware that has an HDMI out might work as well.  Then you'd run HDMI to a surround-sound amp and you're all set.

Another option might be the digital out of the Mac to a surround amp...but that would mean you'd be working software only.

 

if using a Nitris DX, you could also take the ADAT Out, into a convertor(digital-> analog), then use a hardware monitor controller such as a Dangerous SR, the SPL Surround, etc. I have done this before with great results. It eliminates the possibility of introducing multi-frame delay when using consumer grade HDMI gear. 

What is the price range of available, compatible interfaces?

And what are the names of these boxes?

An 8-Channel audio interface (most of them are Firewire) would start at about $500USD. I have mentioned MOTU, I really like them for their excellent MAC-driver support (AVID also lists MOTU as a partner on AMC's product page on their website)

This should do the trick though- it's got 6 outputs:
http://www.motu.com/products/motuaudio/audio-express

About $400USD - I doubt you could get 6-channels out for much cheaper- not without sacrificing quality and/or compatibility...

The OP has an AJA I/O Express- and I think the HDMI out on that box is only video monitoring, and there's no ADAT- he can disembed the SDI audio- but those boxes are more than the OP wants to spend...

This is from the AJA webside:

"Io Express also offers support for both 8-channel HDMI and SDI embedded digital audio"

So maybe there is a cheaper way. I'll do some research and let you guys know.

Thanks again.

I think the website is incorrect- you should look at your user manual (which can also be downloaded off the AJA site...) is seems pretty clear there that multi-channel audio is passed only through the SDI-

I actually called AJA about this out of morbid curiosity- and they confirmed that the IO Express product pages are incorrect and that you should refer to your manual... although it does seem kind of crazy to me that audio would not be passed via the HDMI...

Strange!

I just downloaded the manual and on page 17 I found this:

--------------------------

HDMI Input and Output 

Two HDMI connectors on the Io Express provide input and output of HDMI compatible video and multi-channel embedded audio (8 channels). HDMI v1.3a capability at 30 bits per pixel 

allows full support of the latest 10-bit monitors.

-----------------------------

And if I crank up the volume on my client monitor (connected with HDMI) I can hear the sound.

There is still hope. I guess...

I guess you found your solution : )

Let us know how it works out!

I use the Alesis AI3 analog optical interface to convert the the Nitris DX optical audio output to eight channels of analog audio which is interfaced to a powered 5.1 surround speaker setup.  The Audio settings in Media Composer allow you to patch up to 7.1 configuration.