There should be 2x DACs there, LC7883 that is for CDDA(at least in MegaCD2, seems CDX uses something else) and LC78835 that is for PCM chip.
Regarding LM2676, it did gave me some rolling lines in video, but that got fixed by adding a 100µF cap on the video encoder power pins. I would replace all of the original switcher circuit with some of the modern switchers, they're certainly much more efficient (the LM2676 does not even get warm in my MD2, its the 7805 the heatsink is for, hehehehehehe). I plan on giving my nomad a LM2676, it sohlud give a lovely extra hour of battery life (I want 10 hours, and i'm not too far from getting it).
You can just put my circuit on top of the 315-5684, the volume levels of all signals are right so no adjustments need to be made, just desolder the chip, get rid of some extra components and those series and parallel capacitors everywhere on the audio lines and you're done. You might replace the 4558D with something else, but it needs to have at least 3MHz of bandwidth (I'm not doing too much filtering, and YM output can carry freqs up to few hundred KHz !!!)
I just remembered that there is headphone output on the CDX though, I will look into it first... but I have school coming tomorrow so I cannot do any work on this subject until Saturday.