Three thoughts:
1. I like the frequency converter idea. Even better, if you can mix in a reference frequency for the voltage you're adjusting to, along with the frequency that you're measuring, you could "tune" for the exact voltage by listening to the beat frequency, as you drifted further away you'd hear "wa-wa-wa-wa-wa" and as you got closer it'd change to "waaaaaah-waaaaaaaah-waaaaaaaaaaaaaaaah". Beat counters could similarly be used that would give you a ticking sound like a metal detector or geiger counter. Any old metal detector circuit would get you close. Actually, you could probably hack a cheap metal detector pretty easily to build your circuit.
2. For 12vdc (car audio), I used to use a multi-meter with a buzzer-type continuity detector, but use it to measure voltage. You'd get a pretty good audible indication of voltage as the thing changed tone from a low "buzzzzzz" to "WHEEEE". Yeah, they'd burn out after a while, but they were cheap and easy. I don't know if this would give you the accuracy you're looking for.
3. A Google search on "talking voltmeter" turned up an apparently obsolete Micronta model. Maybe you could find one of those. It also turned up the fact that Fluke played around with a prototype talking voltmeter too, but that it drove everyone nuts so they dropped the whole idea (words of warning to you?)
What kind of voltages are you working with? What ranges of voltages? How accurate do you need to be?