I borrowed Chris’ iMic, and had exactly the same results that I’d had without it. That is, the left channel very quiet, the right channel fine.

Bah.

So I went to Radio Shack and got new wires, and that seems to have resolved the problem, although now the left is slightly louder than the right. And changing the balance doesn’t seem to actually fix that. Oh well, it’s better than before. We’ll see what kinds of results I get now.