Ok. It seems that my problems are running a little deeper than I thought. Previously, soundtracker, using the oss interface and jack, using alsa had been working both in full duplex, though i've just tried these again and now very little seems to be working properly. This is really annoying as I don't think I've changed anything since it was working a few days ago.
After sharing my alsa mixer state with a guy on freenode in #alsa, it turns out that I had a switch set wrong in my alsa mixer. I don't remember switching it over recently, but maybe I did.
Sorry for being stupid. bill