Just for the heck of it, see if a paperclip reset fixes the issue.
If I understand things correctly, isn't there a small amount of soldering required to replace the battery? Maybe something happened there like a tiny dropped solder blob or something, that now needs to be scraped off the circuit board.
Good idea on the reset, but no joy.
There is no chance of a dropped solder blob. I didn't feel competent to solder those tiny wires to the tiny connectors, so I cut the old wires close to the battery, and with the wires on the new battery was able to twist, solder and shrink-wrap the connections completely outside the Karma.
Since the backlight does not come on when I work the switch, I'm guessing that the switch itself has failed, rather than some circuitry that controls the volume.
Here's a hint for anybody else who needs to reassemble a Karma. To prevent breaking the lock switch, pop the slider out of the case before putting the case halves back together. Then, last thing you'll do, is line up the slider with the switch and pop it back in.
Looks like refurbished iPods can be had in the $100 range. Since the only thing I do with my Karma is listen to audiobooks while I'm walking the dog, an iPod would serve my needs.