Well, it seems to have gone away with a new kernel, either that or a reboot.

I've been having some timing madness elsewhere; the attempt to guess the clock
rate of my auich is utterly bolloxed (it gets numbers anywhere from 11KHz to
120KHz, but the real value is 48), so there may be something generally up.

Oddly, if I just force the auich driver to 48KHz, everything actually works
fine.

-s