Rob, don't think you will have much luck getting an adapter for the 8 pin. I'm thinking your best bet would be to get a Digitrax harness, cut the 8 pin plugh off and solder the decoder wires to wires on the Digitrax harness. MRC changed their 1650 decoder (to the 1651) so it would accomodate the 9 pin plug. (It comes with a harness that has an 8 pin plug).
I don't believe MRC makes a 9 pin sound decoder, or for that matter a sound decoder appropriate for an F59.
As for the DD40Ax conversion, is it a Spectrum DD40Ax? I've converted one and can give you some tips/ideas if you'd like.
Tilden