How to build an Audio Spectrum Analyzer/Sensor?
There are three ways of doing this.


The Analog Method. Through the use of Op-Amps or discrete amps made from transistors and discrete band pass filters made from Resistors and Capacitors, you can make a three channel Color Organ. No microcontroller or Arduino necessary.





The Software Method. Through the use of FFT (Fast Fourier Transform) algorithms, you can detect arbitrary frequencies in a signal, and use that to provide the LED signal. Runs completely on your microcontroller. Depending on the library you use, might need a beefier one, but there are ones that run on very low power MCUs.


ATMega8 FFT + LCD (Less powerful than Arduino Uno's ATMega328): https://www.youtube.com/watch?v=drhF_F2Xehg


The pay someone to do it method. You offload the FFT on another chip. The MSGEQ7 and it's sister chips will do the FFT and multiplex the data out. The Old school National Semi (Now TI) LM3914/5/6 can be provide a simple VU meter with LED Strips, and with band pass filters could also do Frequencies. The TI LM4970 will parse the audio and will drive the LEDs directly, or with a transistor LED Strips, but can be configured over i2c.


LM3916: https://www.youtube.com/watch?v=vLYpGfShEBo

LM4970: https://www.youtube.com/watch?v=mCvn9v5jyw4