It's Aliiiive!!
Two additional traces, both checked out fine when I did the initial repair, were damaged between the Z80 RAM and NEO-C1. Added a pair of additional wires to the PCB spanning the broken traces, and the diagnostics ran 100%. Put the original BIOS back in, and hey presto, the sounds are functional again.
Very pleased. I spent half as much on parts as a replacement board, including the hot air station, but it was totally worth it in the end.
Thanks for all your assistance. =)
Now to see if the AES deck I yanked that RAM from can be made to work again... ^__^