Questions about DC offset
You should have a capacitor between the LM324 output and the middle of your voltage divider, which is also going to be the node that is the input on the ADC.  The capacitor will allow the DC offset, while at the same time allowing the middle voltage of the divider to be pushed around by the audio signal.  The divider provides the bias voltage (presumably 2.5V).

Since your amp output has a 10V swing, you will need to choose values for the resistors such that the output impedance of the LM324 cannot push the input of the ADC beyond 0-5V.  (or add a resistor to the output of the LM324 to control the output impedance).  How?  Just consider the output at it's maximum (+5V above the node voltage*) and that going through the output impedance of the LM324 and the impedance of the capacitor in parallel with the top resistor of the divider, with the lower leg dragging it down.

Ra || Ro  in series with Rb --> solve for the voltage at the junction, and that will be the maximum voltage that the input of the ADC.  Do a similar analysis for the lower range.

In all of this you will have to consider the frequency response at the input to the ADC, as well in choosing your resistors and capacitor values.

*/as the top of Ro seeing +-5V above/below the bias voltage, the top of Ro is essentially between 7.5V and -2.5V

Similar to this:


The AC source is the output of the LM324, and the 1000k resistor is the input to the ADC.  The value of the capacitor isn't very important, but you should do a frequency analysis to make sure that the circuit covers the range that you're looking for.  Generally if you are dealing with AC signals you isolate parts of the circuit with a series capacitor, which will not pass the DC component, only the AC/signal portion.  For example, the input to the LM324 should have a capacitor in front of it -- this protects the circuit, if someone were to connect a DC source to it.

The black line is the voltage at the ADC input.  The orange line is a 10Vpp signal offset from the ADC's voltage range.