Found it the problem! Two weeks *not* wasted!
...granted, the current form of the solution is a bit heavy handed.
It's MTRR write-combines on the PCI bus that are causing the huge latency spikes and crippling audio. Ripping MTRR out of the kernel entirely solves the problem (simply disabling the write combines via /proc is temporary. The driver will just reenable them if it can).
Eliminating MTRR support also kills the nvidia driver performance.
In my case, I can live with it if need be; I meant it when I said flawless audio was more important to me than fast video. OTOH, I can't help but think there's a happy medium here that doesn't involve either abusing the PCI bus's latency handling or backing off so far as to make video unbearably sluggish. I mean, we're talking back to the days of unaccellerated Oak cards here ;-)
Hopefully now that there's a glimmer of hope the nVidia folks will comment...