WetGeek I guess k3b knew what it was doing better than I did.
Actually, it didn't. In the encoding options, .ogg had been selected as the default. The first time I tried this, I selected .mp3, but in my haste I'd forgotten to unselect .ogg. So both formats were produced, as in the image above.
If you want to try this method, there's a little k3b gotcha you need to know about. For my second attempt, I selected the files to write using k3b, and it balked, telling me that remote shares are not supported. Then I remembered that the first time, I'd selected the tracks in Dolphin, right-clicked, and selected Create an audio CD in k3b from the context menu. For some reason, that works where selecting the files from k3b itself doesn't work. Go figure!