OK, so I didn't even build the last PCB. The SPI bus is overloaded by having both DAC and input device (SD card or SPI EEPROM / SRAM).
I spent a long time trying to stream audio from the SD card on the SPI bus to a RAM buffer and then back out the same SPI bus to the DAC. Basically, interrupting transfers from the SD card to make regular (every 22us) audio SPI transfers doesn't work. Or, at least I can't make it work.
I investigated the USART MSPIM capability (present in every Arduino AVR) as an alternative, and found it works great! AVR USART MSPIM Description.
So, I built it into a new PCB which is coloured blue to avoid confusion.
WOULD LIKE TO HAVE YOUR INPUT ON MEMORY OPTIONS, PLEASE...
Available SPI Memory Options: Pick 2 of 3 (or 2x of one type).
EEPROM: 1Mbit - non-volatile, slower to write, cheap, large capacity
SRAM: 1Mbit - volatile, fast, cheap, large capacity
FRAM: 64kbit - non-volatile, fast, limited storage capacity