Okay, never mind, my previous two posts were posted under the influence of some severe misconceptions.
I thought the green bar indicates maximum possible volume, but it actually gives a live indication of actual volume. I didn’t realize this, in part, because whatever the problem was, it was apparently causing the computer to playback (or think it was playing back, since I couldn’t hear anything) a steady, constant stream of sound at a constant volume. This meant that whatever setting I put the volume at, the green bar would stay frozen at a corresponding lower point on the volume indicator.
Well anyway, now that I’ve pushed “default,” whatever that did, everything’s behaving normally, and there is a green bar, behaving as it ought to behave.