(I think 0-20,000 hz)
Not very realistic. While there is some energy in music above 10KHz their is very little of it.Here is what I did with an MSEQ7https://vimeo.com/167914646
Sampling, calculating and driving LEDs at 40kHz is probably going to be an issue. I would consider using a more powerful processor, or use something like an MSGEQ7.
Is there another processor that would work in this situation,
i need 16
you could use 2 MSGEQ7's, one for each channel, and interpolate between the two lowest bands, for example
There is a clock oscillator pin, but I'm not sure what frequencies to oscillate it at.