The MEGA65 currently indeed has two soft SIDs and a left and right digital audio channel. You can also set a register that tells the MEGA65 to talk to external SIDs on a cartridge. The mixing circuitry for the audio would be part of that cartridge (which doesn't yet exist).
As for DMA or FIFO assistance for digital playback, this is of course quite feasible to do, and not that hard, but we haven't done it yet. Also, as LGB has said, at 50MHz, the cost of pushing a sample into the digital audio output lines, on every raster line if you so wish, is pretty cheap. That said, we will likely add at least some sort of digital audio FIFO.