I'm not sure if I can see the picture correctly.
Is that negative 514? The ADC doesn't read negative and with the 2nd circuit there is no bias so you shouldn't subtract. But either way the readings should change with an audio signal.
Try with just the 100K resistor first and no audio connection. You should get readings of zero (perhaps with a little noise). Then you can add the 10K resistor and nothing should change.
Without the any audio signal, connect the audio circuit to the 10K resistor. If it still reads zero you probably don't need the capacitor. You probably don't really need the diode either, so go-ahead and try some audio.
When/if you add the diode make sure it's connected in the right direction. If you connect it the wrong way you'll kill most of the positive half of the signal and your readings will be limited to about 40 (or around 100 with a regular diode).
With John's circuit you won't get any readings until you are above the diode's forward voltage (~0.2V with a Schottky or ~0.6V with a regular silicon diode) so you won't read anything with low signals. This may be OK if your signals are hot enough. (Or, there's probably a way to add another resistor for a slight bias.)
[quote]Btw is a peak detector something you have to diy or can you buy it as an IC? Because it looks kinda complicated to build.[quote]It's DIY. (lthough there are some microphone boards with one built-in.)
To me, the biggest pain is that it normally requires dual power supplies, although a "rail-to-rail" op-amp might be OK at 5V. I use higher voltage supplies to insure the full 0-5V range, and then I add another resistor & diode for over-voltage protection.
Other than that, if you look at the schematic, R2 is shown as zero-ohms so it can be eliminated and replaced with a wire. And if you're not using R2 along with R1 to get gain, you can also leave-out R1. So really it just requires an op-amp, a diode, a capacitor. and a resistor. Actually.... With an audio signal, which may not have a DC "path" to ground, you need one more "pull down" resistor on the op-amp's + input (10K-100K to ground).