A resistor based voltage divider won't work, because of Ohm's Law again. Eg. if we used even 200/100 ohms for it, Mr.Ohm says Voltage must equal Resistance * Current(I), or V=IR.

So 3.3V/200ohms = max current of 0.165A (16.5 milliamps). At first glance, that might even be enough current, but sadly the WT32i also has (internal) resistance that must get factored in.

The result being, that to use a resistor voltage divider for this means really small resistors, and therefore high current and very HOT resistors. As in, melting, unless you get special ones.

So, a resistor divider won't work here -- it's really only useful for tiny current draws (Ohm's Law again).

The simplest solution is to just feed 5V into the USB connector of the Arduino. That's how I've been powering it in the car: USB cable from the USB ports in the car to the Arduino board. In your case, it would be 12V to your PSU, and then a chopped-up USB cable to transfer 5V from there to the USB connector on the Arduino.

Note that with I2S digital audio, you no longer need a separate/special PSU to get nice audio.