To clarify, VIN should be 7-12 V and is regulated to 5 V by an onboard regulator. If you provide 5 V yourself, connect it to +5V, not VIN. See here:
(that’s for a Nano, but Leonardo has a similar power supply, using an NCP1117 that is similar to the LM1117 and AMS1117 regulators mentioned there, with a 1.2 V drop)
The input pins have protection diodes to deal with input voltages above and below the supply rails, but they cannot handle a lot of current, so unless you have current limiting resistors on the way in, over- and undervoltages may fry them.
Fredrick, Thank you for the excellent and clear information. That’s a lot better than what I was trying to say. Appreciate it. I can say that it is possible to send CV into an Arduino and using various libraries have it act as a VCO and its been working pretty well for me! The issue has only that the Mozzi library spits out a PWM basically on its output pin that has to be integrated into an actual wave and it took me some back and forth to get a high quality signal out. Mozzi docs have a suggested resistor capacitor combination but for some reason it didn’t work for me out of the box and I had to fiddle.