Signal volume LED Array
An electronic VU meter is show below so you have an idea how to &quot;rectify&quot; the audio before it goes into the chip.

I have circled the input where you connect your audio signal and the output is circled where you feed the rectified audio in the next circuit. Please note that I would have suggested this type of VU meter using the chip shown but because you can only use op-amps or simple components you'll have to use comparator circuits as mentioned elsewhere in this question. Alternatively this is a simple idea using diodes and transistors: -

Hopefully this is simple enough to mafe an effective LED sound meter. You can omit the opamp circuit - go straight in with the output from the rectifier circuit via a 1k resistor. As for power, the LED circuit shows it running from 13.5V nominal - it should be fine to run it at 12V or possibly a little lower.