My sound card is now substantially complete, and I can continue working on system assembly code to better utilize it. I have the sound card installed in the system with the system sending control commands to the audio co-processor.
Recap of the sound card's key features:
Six YM2149 programmable sound generators (PSGs) with three analog channels each.
Dedicated audio co-processor, which is a 65816 on the sound card.
Onboard 512 KB flash ROM for media.
Onboard 64 GB SD Card for media.
Can play audio with a simple command from the host system's 65816 and free the host system to work on other tasks. Communication is handled with dual-port SRAM and interrupts.
Audio co-processor (65816) is running at 10 MHz. It also runs fine at 12 MHz, but I'm leaving it at 10 MHz to have a little cushion -- and I don't need 12 MHz to play audio. The PSGs are using a 2 MHz oscillator for tones. The host system runs well at 12.5 MHz; at 14 MHz the video card I/O begins to have misses when all other I/O cards are also installed (I'll work on this to see if I can get it to 14 MHz with the system fully populated). Update: I changed my bus resistors from 1K to 560, and the system now runs fine, fully populated, at 14.16 MHz.