Ah-hah.

Found one problem: In the one that was playing back silent, there was a separate volume control in PowerDVD, and for some reason it needed turning RIGHT up, and even then WAVE output was not huge.

On that machine, the other drive is CD only, so one machine down (except that I have an unidentified device that Windoze thinks is the PCI bus, which it isn't because if it was there would be no sound output at all....), two to go.