I'm unsure if there are any digital decoders out there right now that could directly interface with the S2 (some do directly interface with some, but not all, SDRs -yet), however if it were me, I'd get copies of MultiPSK, Sigmira, Sorcerer and possibly Rivet. I would start with MultiPSK as it has its own dedicated Yahoo group. Do one at a time so you don't get overwhelmed by too many issues all at once.
It may take some trial/error to get them working - I would likely ask on some associated Yahoo groups and see what develops (along with MultiPSK the UDXF- Utility DXers Forum- would be another likely spot along with any Yahoo group that supports this SDR or manufacturer).
You can find the links for these in the page link in my sig.
Mike