
1. DAC is 8bit, not 9bit.
2. DAC seems to be one not only for 6 channels, but for left/right output too. But I don't get it: oscilloscope shows channel synchronization:

Perhaps they used the capacitance to store the current signal level. DAC place at upper right corner on picture. All analog pins can easily be traced to him.
The first step is to identify all the blocks in the picture. Interestingly, below the center at right side there are some matrixes, perhaps this is the operator unit and the calculation tables for the level of the envelope and sine. And they're seems 14-bit. On the left side above the middle is likely array of registers.
Learning, thinking and posting here.
PS Any part of picture can be recaptured with better quality. Source PSD file was 1,5GB. :3