Low power envelope follower + MOSFETs

[egasimus]

I'm working on one of those circuits which use an 8-pin microcontroller to fade some RGB LEDs to different colors. I need the lights, however, to pulsate to the beat of any music that might happen to be playing - basically, a VU meter for ambient sound.

I've decided to use a small electret capsule to pick up the sound, and that's where my questions begin. First of all, I need a (really simple) envelope follower; and, since the circuit is likely to be running on as little as 2.5-3V of AAA cells, I need one which can work at such a low voltage, and without drawing a whole lot of current, either. CMOS inverters come to mind.

My LEDs are driven using PWM by some MOSFETs. I considered reading the output voltage of the envelope follower with the chip's analog-to-digital converter, and then modulating the LED brightness in software, but that looks like a lot of messy code to implement, and I consider myself too inexperienced to attempt this. So can I instead modulate the source voltage of the MOSFETs which drive the LEDs, and, if I can, how do I do it?

Note that the different colors of the RGB LED also have different voltage and current requirements... damn.