That function is not allowed as per the CECB rules. Having that kind of output is classified as an "upgrade," which can't be done by a coupon eligible converter box. You'll have to use a set of sustained frequency-based surround sound simulators (those old mid-80's boxes that turned mono sound into "theater" sound. They work by delaying a frequency's sound output across two speakers making a particular voice or sound appear to come from opposite directions.)
You will need to get a non-eligible device which has that particular output. Panasonic DMR EZ28/ EZ48 has that SPDIF connector for the sound output.