According to the 2012 Electrical service manual, there are 6 resistors in series. At each junction, there is a switch that causes the remaining resistors to be bypassed. I don't know exactly what is being sensed (voltage or current), but it goes to the APIM, Accessory Protocol Interface Module. Likely what happens when you press one of the buttons and bypass the associated other resisters, is a unique current can flow to the APIM. The current goes through a reference resistor and therefore creates a unique voltage for each button press.
As it turns out, the "
Voice/Mute button, as it called in the manual creates a resistance of 53.6 ohms, or the most current flow and the highest sensed voltage.
Vol- is the next button and causes 142.3 ohms, or the next highest current and the next highest sensed voltage.
Vol+ is the next button in the line up
Seek + is the next
Seek - is the last...with the highest resistance, lowest current, and lowest sensed voltage at the APIM.
So here is the observation....if any of the buttons actually do something (as you said they do), then the Clockspring and the connections are all good. If any of the other buttons on the lower right quadrant of steering wheel are working as they should, then the APIM is likely ok too. So if just the Voice button isn't working right, then perhaps the actual switch connection has some resistance to it causing the APIM to think it is one of the SEEK buttons which are further down the chain. If all the buttons do something different, then I'd suspect a loose connector somewhere.
There is a diagnostic mode for the upper steering wheel switches, but I couldn't find any mode to test the lowers. Anyway, you might try fiddling with the switch to see if it ever reverts back to the way it should work. If that happens, then it might just be a switch issue...not that a switch issue is good news.