The SPI bus on the Arduino platform (including the Goldilocks Analogue) is overloaded with activities. Driving the uSD card requires tying up the bus for many hundreds of milliseconds, and this conflicts with the need to output regular audio samples.

Moving the DAC to the spare USART1 on the ATmega1284p MCU was the best answer.

Using Master SPI Mode on an ATmega USART