I'm using an op-amp with a full wave rectifier and some gain to tranform the audio signal into a 0-5v signal to feed into the arduino/atmega adc.
Then for the VU meter its a simple sampling and mapping for the 75 different values that I can show with the lcd, altough I know that a real VU meter as a logaritmic scale, when I have some time I will try to implement a real VU meter algorithm, for the FFT/spectrum analyzer I'm using elm-chan FFT engine, for the 64 points FFT and all the others variables used in the fft and vu meter functions I'm using less than 512bytes of ram and still under 3,5k of code.
The lcd is updated at about 14Hz using a timer interrupt routine.
Yes, I can show the source code, but almost all the comentaries are in portuguese, but I can translate it if some one requests.
This is the schematic for the input filter:
The filter was done with help from Macetech that show the schematic from its Shifty VU shield, as I also read this:
http://sound.westhost.com/appnotes/an001.htm
The lcd lib that I used is this one:
http://www.jump.to/fleury
And this is the FFT lib:
http://elm-chan.org/works/akilcd/report_e.html