I think I may have been able to fix it!!!
I plugged the iPod nano up to iTunes and restored it, reloading all of the iTunes content (songs and audiobooks). At first, I had the same issue with the voiceover playing endlessly. After I restored it, I turned off the voiceover in all of the locations:
1. Settings, General, Accessibility, Voiceover - turn to off. Also, turn off "speak hints".
2. Fitness>spoken feedback>off
3. iTunes - there is a place you can turn off the voiceover.
4. For grins and giggles, I also turned off the "shake to shuffle" feature on the iPod (it shakes a lot during a workout, so that could contribute).
After I restored, the same problem happened until I turned off #2 above. When all of those locations were turned off after a "restore", it stopped. I had nothing but beautiful music. I've had two good workouts with music since then and it's still working normally.
If you're having this problem with the uncontrollable voiceover, I would suggest doing a restore (will require you to reload all of your iPod content). Then go through each of the above locations and verify that they are all still turned off. That seems to have worked for me. If it starts again, I'll let you know.