I've been busy trying out, reading about and understanding different aspects of using PWM as a means of getting audio out. There are some hidden treasures and possibilities, as well as pitfalls, that I really hadn't thought about before.
Having tried different solutions, I've now settled for a simplistic approach of using stereo 8-bit phase correct PWM. It sounds good enough without lots of complexity. Now onwards with the serial comms to send data to the soundchip.