These test points (TE4, TE6 and TE7) would be much easier to solder onto, but could they be connected to Arduino safely (within current limit?) and would it be easier to decrease this voltage to the 5v for monitoring instead of using the IC1's LED-level output?In any case, you will need to reduce the voltage*. Your circuit has signals between zero and around 18V. The Arduino can be damaged with voltages above 5V.
Reading voltage pluses
with a meter only gives you a rough idea of what's going on, and different meters will "average" differently...
Looking at the schematic, I don't think those test points are going to give you a good signal (i.e. it doesn't like it will go close enough to 0V).
The outputs of U1 (pins 1, 7, 8 ) are probably the best place to pick-up the PCM signal (which will switch between around 0 and 18V). A voltage divider
(2 resistors) can be used to knock-down the voltage. I'd use 10K as the aproximate total resistance for your voltage divider. 2.7K and 7.5K are standard values that will probably work.
Then, if you are going to use an Arduino analog input, you'll need a low-pass RC filter
to convert the PWM to variable DC. I assume a time-constant of around 0.1 Second should work. And, again, I'd start with about a 10K resistor and you can calculate the capacitor value. (The resistors in the voltage divider will interact with the filter to an extent, but you'll probably be experimenting with different values anyway.)
I have to agree that the Arduino is probably overkill! But, it does give you the ability to do anything you want with those 3 color-signals!
* On the Arduion's inputs, we have to be concerned with voltage. On the outputs, we have to be concerned with current. The relationship between current, voltage, and resistance (or impedance) are is described by Ohm's Law
. The very-high input impedance of the Arduino (under normal conditions) means that current flow into it is very-very low.