digital to analog converter

You’ll need an Toslink optical cable, glass or plastic depending on your budget, and a DAC with an optical input. For output from a TV, a relatively inexpensive DAC should be fine, have a look at models from Schiit and Topping. Then run a pair of R/L analog cables from the DAC to one of the analog inputs on the Mac. On the TV you’ll need to go into the audio section of the settings menu and choose the optical output in place of the TV speakers. 

Best answer:  Replace it with an HDMI 2 capable processor or AVR and use HDMI/Audio Return Channel.

Cheapest answer:  Get a Topping 2 channel DAC and optical S/PDIF cable.

thanks to each for advice.

See if this firm suits your budget:
Lifatec

S Video?
Who you crappin’?
The only valid answers you’ll get about what is possible will come from Mac.
Personally I wouldn’t bother.
I hardly ever mess with watching surround sound on my Sony TV’s.
Super Bowl excepted.
Good luck with that.