The 1K resistors in the circuit are capital so that the LED's about-face on at altered audio levels. There is no acumen why you can't change these resistors, although annihilation aloft 5K may account some of the LED's to never about-face on. This ambit is calmly abundant with added op-amps, and is not bound to use with the LM324. Pretty abundant any op-amp will assignment as continued as you attending up the pinouts and accomplish abiding aggregate is appropriately connected.
The 33K resistor on the schematic is to accumulate the arresting ascribe to the ambit at a low level. It is absurd you will acquisition a 33K resistor, so the abutting you can get should do. The amount of this resistor may charge to be changed, so it is best you breadboard this ambit afore absolutely amalgam it on PCB. The ambit in it's accepted anatomy will acquire band akin inputs from sources such as the aux out on a Hi-Fi, all admitting could be calmly adapted to acquire apostle inputs.
The audio + is affiliated to the capital absolute rail, while the audio - is acclimated for arresting input. The 50k pot can be acclimated to alter the acuteness of the circuit.