Sampling stereo audio at greater than 40 Hertz using arduino
If you are only interested in getting a reading of the sound intensity,
then 8&nbsp;kHz may be enough. You can also convert the signal to mono
using a pair of resistors. If you want the Arduino to do other stuff
while taking the analog readings, then you should forego analogRead()
and access the ADC directly. This is more involved that typical Arduino
programming, as it involves configuring the ADC at low-level, in “free
running mode”. In this mode the ADC operates in an autonomous way, and
send you an interrupt every time a sample is ready.

I have written a program that does something similar: it samples an
analog input at 9.6&nbsp;kHz (104&nbsp;µs per sample) an periodically
sends the reading through the serial port: sound-meter.ino. You can
probably adapt it for driving LEDs instead of the serial port.