Usually, the discreet channels of audio on SDI are just channels of audio....like different language tracks, or separate music from effects and dialogue...used for mastering to give the broadcaster options, or if you are editing trailers for upcoming films and want the intended final music separate from the dialogue on your selects. These discreet tracks will each go to however many tracks your tape deck may have, or just pass through as their own tracks. It looks like your Deckling HD pro also comes with SPDIF audio out, so yes, you will need some sort of D/A converter or decoder. There are components for this, some for HiFi others for pro audio...with wide price ranges from $300 - several thousand for high end units. You could, of course, use an A/V receiver's digital coax input, and then maybe tap the 'record out' RCA jacks to your mixer or what have you. The SPDIF will only output 5.1 surround if your source sound is encoded that way...like a Dolby Digital AC-3, or if a certain sound editing program has 5.1 output via the digital out. Otherwise, all it will be is a summed stereo output of whatever's on your timeline, although I'm not familiar with realtime 5.1 editing.
There is no standalone D/A converter that I know if that will otuput the 12 discreet audio tracks on the SDI output as separate analog outputs. If there was, it would likely be quite costly. If you're just looking to be able to monitor audio while editing out through your Decklink card, I'd look into integrating an AV receiver's internal D/A converter, an older Technics converter like this:
and take the mini-plug analog audio outs via a mini-stereo to analog RCA adapter.
But again, if you're just doing basic stereo audio tracks while editing in FCP...be it 2 or 12 tracks, the SPDIF will output a summed stereo of everything, with no way of separating them like with the SDI.