Neopixel LEDs Affecting Theremin Pitch

I'm attempting to make a battery-powered Theremin using the OpenTheremin V3 shield, and part of my project has been to include a couple LEDs on the back that light up depending on the volume. I used Neopixel LEDs, as they would take up only one digital pin on the Arduino, but I've been running into an issue. For each additional LED lit up, the pitch jumps up. I tested with changing the brightness of all of them variably at first, which caused wild changes in pitch, but now I've tried with turning them on one at a time and I've found that the pitch increases uniformly with how many LEDs are turned on. Having the LEDs always off or always on works fine, however. I was wondering if someone with more knowledge on how theremins work could tell me whats going on and if connecting the Neopixels to a separate power supply or adding a second micro-controller to control the LEDs would fix the problem.

Probably, OpenTheremin oscillators frequency depends on power supply voltage.
Try powering leds from separate battery or separate regulator.
BTW, is OpenTheremin connected directly to battery or via regulator. If directly, try using regulator.

Yeah...we're so used to LEDs being "low power" compared to other illumination that we fail to remember that they draw quite a lot of current compared to other electronics! All the open theremins are highly susceptible to power fluctuations. 

I don't think there is any reason that the Uno can't handle the prcessing, so I would try using a second battery, dedicated to the NeoPixel power. If that doesn't work, then my next try would maybe be towards using some sort of low power optoisolator?