We seem to have solved the "missing right channel" problem. It
was, in fact, a software problem. We were running Fedora 4, and
when we installed the latest patches on March 31, that's when the
right channel vanished. We tried downgrading to the version of
the kernel and ALSA as of three months ago, and that didn't fix
it. But, Jonathan took all the sound cards home and tried them in
his machine, and they all worked fine there. He was running
Fedora 5. So we upgraded to that, and the problem went away.
That's right: upgrading to the latest FC4: breaks the world.
Giving up on FC4 and going to FC5: un-breaks it. Nicely done, guys.